I finished a quest with Bairdotr and gained 180XP.
8000 (starting) XP + 180 (Quest) XP = 8180XP

After that, I exchanged Bairdotr with Wolgraff.
Wolgraff gained only 10XP. 8000 (starting) XP + 10 (AutoLevel) XP = 8010XP

Is it a bug?
How do companions auto level?