Elevation problem

Yesterday I did a standard route around the house and the altitude was different than in reality. Karoo showed about zero with a drop below zero (!). I live at an altitude of >90m. Today I checked the "altitude calibration" and it shows 93m. So it's ok, but what was yesterday? The weather was good, sun, pressure 1002hPa. Min elevation registered by Karoo was  -10m, max +30m.

The same ride from my watch. Min elevation 81m.

Similar ride a month ago

What's the matter?

I checked a few historical rides and the differences are big.