Where is my CH going?

[botanica37]

Help me figure this one out: for the past month the CH has been stable at 280, including this Monday and today it is 250. Haven't backwashed at all or drained the pool in the meantime. There was a bit of rain, but no more than an inch...here is the full test results:

FC 2.5
CC 0.5
TA 90
CH 250
PH 7.8
CYA 30
T 77

I have no leaks from anywhere, I am not losing any water...and I double checked the result :?

 

[JasonLion]

There are a couple of possibilities, you had more water replacement than you thought, there was a testing error either before or now, or the PH spiked up and you had just a little calcium scaling.

 

[257WbyMag]

Rain, which will have no calcium in it, can reduce your CH in fairly short order. It can also do the same to your CYA and TA. Recheck everything to rule-out the possibility of a testing error and then you should have your answer.