values continue to benefit from consumers spending additional time and money repairing and maintaining their vehicles in response to the COVID-19 pandemic, economic environment and new and used vehicle scarcity. Average ticket values also benefited from increases in average selling prices, on a same-SKU basis, as compared to the same period in 2020, driven by increases in acquisition costs of inventory, which were passed on in market prices. The decrease in transaction counts for the three months ended September 30, 2021, was driven by a challenging comparison to the strong DIY customer transaction counts in the prior year, partially offset by positive transaction counts from professional service provider customers in the current period. Improvements in transaction counts for the nine months ended September 30, 2021, were primarily due to prior year headwinds to traffic from the initial COVID-19 stay at home orders in 2020 and business restrictions, which resulted in immediate pressure to transaction counts for both DIY and professional service provider customers. Continued market reopening and recovery plans, ongoing government stimulus, favorable winter and spring weather conditions and the new and used vehicle scarcity positively impacted our customers’ willingness to perform or invest in maintenance on their vehciles, which benefited transaction counts in the nine months ended September 30, 2021.
We opened 30 and 146 net, new U.S. stores during the three and nine months ended September 30, 2021, compared to opening 30 and 153 net, new U.S. stores during the three and nine months ended September 30, 2020. As of September 30, 2021, we operated 5,740 stores in 47 U.S. states and 22 stores in Mexico compared to 5,592 stores in 47 U.S. states and 21 stores in Mexico at September 30, 2020. We anticipate total new store growth to be 165 to 175 net, new store openings in 2021, and 175 to 185 net, new store openings in 2022.
Gross profit:
Gross profit for the three months ended September 30, 2021, increased 8% to $1.82 billion (or 52.3% of sales) from $1.68 billion (or 52.4% of sales) for the same period one year ago. Gross profit for the nine months ended September 30, 2021, increased 15% to $5.29 billion (or 52.7% of sales) from $4.61 billion (or 52.6% of sales) for the same period one year ago. The increase in gross profit dollars for the three months ended September 30, 2021, was primarily the result of new store sales and the increase in comparable store sales at existing stores. The increase in gross profit dollars for the nine months ended September 30, 2021, was primarily the result of new store sales and the increase in comparable store sales at existing stores, partially offset by prior year gross profit dollars generated from one additional day due to Leap Day. The decrease in gross profit as a percentage of sales for the three months ended September 30, 2021, was due to increased distribution system costs and a greater percentage of total sales mix generated from professional service provider customers, which carry a lower gross margin than DIY sales. Increased distribution system costs were driven by the significant increase in volumes over the past year, challenging labor markets and ongoing global logistical supply chain pressures. These headwinds were partially offset by a benefit from selling through inventory purchased prior to recent acquisition cost increases and corresponding selling price increases. We determine inventory cost using the last-in, first-out (“LIFO”) method but have, over time, seen our LIFO reserve balance exhausted, resulting in a LIFO inventory value above replacement cost prior to September 30, 2021. Our policy is to not write up inventory in excess of replacement cost, and accordingly, we had effectively valued our inventory at replacement cost, resulting in a benefit when selling prices increase as we sell through this lower cost inventory. During the three months ended September 30, 2021, our LIFO reserve reverted back to a more typical credit balance due to recent, significant inflation in acquisition costs; as a result, we anticipate a diminishing benefit moving forward from the final sell through of inventory valued at older, lower replacement cost. The increase in gross profit as a percentage of sales for the nine months ended September 30, 2021, was due to the benefit from the sell through of inventory purchased before acquisition cost and selling price increases.
Selling, general and administrative expenses:
Selling, general and administrative expenses (“SG&A”) for the three months ended September 30, 2021, increased 11% to $1.06 billion (or 30.6% of sales) from $955 million (or 29.8% of sales) for the same period one year ago. SG&A for the nine months ended September 30, 2021, increased 12% to $3.04 billion (or 30.3% of sales) from $2.73 billion (or 31.1% of sales) for the same period one year ago. The increase in total SG&A dollars for the three months ended September 30, 2021, was the result of additional Team Members, facilities and vehicles to support our increased sales and store count, combined with the unsustainably low level of prior year operating expenses resulting from our cautionary approach and stringent expense control measures taken in response to the onset of the COVID-19 pandemic. The increase in total SG&A dollars for the nine months ended September 30, 2021, was the result of additional Team Members, facilities and vehicles to support our increased sales and store count, increased incentive compensation for Team Members resulting from our increased sales and the unsustainably low level of prior year second and third quarter operating expenses, partially offset by prior year incremental SG&A expenses incurred from one additional day due to Leap Day. The increase in SG&A as a percentage of sales for the three months ended September 30, 2021, was due to deleverage of store operating costs, resulting from the unsustainably low level of prior year operating expenses. The decrease in SG&A as a percentage of sales for the nine months ended September 30, 2021, was due to better leverage of first quarter store operating costs on strong comparable store sales growth, partially offset by an increase from the unsustainably low level of prior year second and third quarter store operating costs.
Operating income:
As a result of the impacts discussed above, operating income for the three months ended September 30, 2021, increased 4% to $755 million (or 21.7% of sales) from $725 million (or 22.6% of sales) for the same period one year ago. As a result of the impacts discussed