In engines vs engine game, both think "optimistically": that they are in better shape than what their opponent thinks the evaluation score is I observed in many Tcec games of season 10 (and possibly repeats trough seasons) that white evaluation score of a position is always greater (not in terms of absolute value, but in terms of the ">" math relation) than what their oponent thinks. 

Now, the engines don't both evaluate the same position, they evaluate only their own moves, so to compare, I'm comparing with the previous and following move, in average. 

You can see for yourself that in the Tcec link at the beginning, down there is the "Evaluation" graph. And the white "curve" is over the black in most game most of the time. 

What causes this phenomena? 

Is that after making a move the position is improved for our side and the evaluation changes accordingly? 

I don't think, evaluation shouldn't depend so much on side to move. Is that a engine always do the things he likes, which the opponent maybe likes but maybe not and thus differing in the evaluation?
It doesn't really matter what the absolute scores are, they really only serve to compare moves internally so only their relative values matter.

With one exception: positions that are evaluated at a draw get 0.0 by all these engines.

So this "optimism" factor is tuned to create a difference between a position that is merely equal and one that is a draw; if it's set to +0.1 for an equal position, then a position must be worse than -0.1 before taking a draw becomes preferable.

The usual name for this is the "contempt" setting. The current TCEC Rules say that the engine's default setting is used, so it can be set by the programmers before the tournament, and updated versions of the program can be entered for later stages of the tournament.

I think that in stage 1, an engine that has too many draws won't qualify for stage 2, so I'd expect some slightly higher than normal contempt, and this effect should be clearly lower in later rounds. But I'm a complete layman and this may be wrong.