Unit's level up bonuses in Ice and Fire

[jahes64]

Hi everyone
I Already had played WotN with DLC deactivated.
Now after some time, i went and started a new game with the DLC activated, played the game for a while, but unit's level up feature confused me a lot.

Thing is, in middle of a certain fight, one of my units got a level and because of it, some additional bonuses as well. But since i was not content with result of the fight (due to unit loss), i reloaded my save and fought the same enemy stack again and the same exact unit got a new level again, but this time the bonuses are different compared to previous time!
How comes? These bonuses are randomly chosen, or are based on the way i use the unit in question?

Many thanks in advance for any info given.