Items price change when stack size changes

There seems to be this issue where for example i have 5 fragments i want to sell for 100 chaos each. then i farm and bit more and the price of the fragments go to 110 chaos each. i then price my 7 fragments for 110 chaos each but once ive sold 2 and back to 5 fragments, the price gets reajusted automatically to 100 chaos each as if it remembers that at 5 fragments it was 100 chaos.