CYA reading lower than my dosing math says it should be

Running a spreadsheet since May, FC 4.5, pH 7.6, TA 80, CYA 45 as of this morning tested on the K-2006. Problem is my calculated CYA based on what I have added this season says I should be closer to 60 by now. No big splash outs, no backwash on a sand filter (cartridge), just normal top offs for evaporation in a screened enclosure so not even wind blowing water out. Only thing I can think of is the sun shelf water getting slightly different exposure and somehow skewing where I pull my sample from, but that seems like a stretch. Anyone seen their actual CYA run consistently under the math, and if so did you ever find the actual cause or just start trusting the test over the formula. Not panicking about it, just bugs me when the numbers don't reconcile.