It's possible, that one is not that firm into the loading case (had that one time or another) and just doesn't load properly. Those magnets don't hold every bump. 😏
Noice Cancelling! Noices come from different directions, so one AirPod is more active then the other.
Phone calls etc. The system decides, which AirPods Mic is used - and can switch it in real time. So there can be one used longer or more often than the other.