If your average pool depth is 4.5 feet (which is usually the case if your shallow end is 3 feet and your deep end is 6 feet), then a CYA at 100 should get reduced to around 80 (even if your deep end is 8 feet, the drop should be similar). 100*3.5/4.5 = 78; 100*4.5/5.5 = 82.

The CYA test is hard to measure at the higher CYA levels as you can tell from the closely spaced lines on the tube at higher CYA. I suspect measurement variation or error more than anything else. Either that or the CYA was actually more than 100 (since 100 is the highest the turbidimetric test measures).

I would just do another drain/refill and you should see a drop.

Richard