Hmm. Not expected. The motors have exactly 200 steps per rotation (not 200.01), the pulleys have exactly 16 teeth. So they should be able to move exactly the correct number of teeth over enormous distances.
The only “analog” factor is the belts. They are supposed to have 2mm per tooth. They are reinforced with fiber inside the rubber. So they are hard to stretch beyond their intended length.
But I am not sure what the actual tolerances are for the belts. They may never have been intended to be that accurate over 8’.
There is a cheap fix though. You can just adjust the steps/mm in the firmware and it will know to add a few steps when traveling that far and it should be able to close that gap of 6mm.
6mm/2438.4 = 0.0025
100*1.0025 = 100.25
So changing your steps/mm to 100.25 should be close. It will add 0.25mm for every 100mm you travel.