Retirement tax questions

Yes, I am a publisher. I have a garage full of books that I sell. In the Income&Expenses>Inventory section, I have entered my beginning 2018 inventory as 38,635, and my end 2018 inventory as 31,615. A difference of 7020. Then, on the next screen, I entered 7495 as my Cost of Purchases. Then on the next screen, it says that COGS is 14,515. COGS should not include the 7020. Why would the difference between the value of my inventory have anything to do with COGS?