I think there are many possible reasons:

1. Ukrainian Artillery shelling and they were not yet able to confirm deaths of the crews.
2. Ukraine got through the frontline somewhere resulting in Artillery being left behind. This could also mean that counting would be paused for a while and we would get huge numbers later.
3. Russians just leaving their stuff behind and running for their lifes.

There are probably even more explanations but I’m sure we will know soon.