Comment by holowoodman

Yes, for larger volumes of rain, that sawtooth pattern is visible and can be corrected for, or even ignored and averaged out. But for low volumes of rain, there will only be one tick. Usually you don't know which side of the bucket that tick was on, because most sensors just count the tipping of the bucket, not the direction or resting position. So for the one-tick-per-day case, you will very often not know whether to count the "large" or "small" bucket amount.

Calibration will also be drifting off for dirt accumulation and evaporation. Different fill states of each bucket lead to different evaporation in each bucket (smaller will evaporate faster, deviating towards even smaller amounts), and different resting times lead to more dust and algae settling in the bigger bucket, making it even heavier.