My ally demesne levy size bigger than maximum, how he achieved it?

A possible explanation could be that Petar's levy capacity got reduced for some reason while he already had all his troops risen.

Possible causes for losing levy troop capacity could be that he lost a couple titles from his demesne (to other realms or to his vassals) or lost a couple points in martial skill (maybe he just recently lost a trait or temporary effect which gave him a bonus to martial?).

Few possible explanations.

1. He has raised levies, but then lost something (titles, martial points, etc.) that have brought max down without losing troops directly.
2. He has mercenaries or personal guard currently active, that are bringing his total troop count above max levies.