If you get enough rain to make the pool overflow somewhat, you are replacing pool water with water that will dilute all values (TA, CH, CYA, FC).
Over a normal wet winter here I will typically see CYA decline rather a lot, like 15 or 20 points. TA can be kept up with just a small amount of tap water since my tap water has TA of like 340-ish.
Also, rain will typically add organics to the pool with a bit of dirt and leaves from my badly sloped flower beds nearby and the leaves and stuff that blows in. So that uses up FC also.