Heroes 5.5 WarGame Edition - Tactical Edition.

There's a distinction: ghosts are light flyers, vampires are heavy (but their base isn't a flyer, it's infantry). In general, the heavy class (heavy infantry) is a class of weaponry, not the weight of the creature. Yes, the griffin is at level 6, considering the stat increases (an increase of 6 on a level 6 unit seems too extreme). And in principle, if it were necessary to scale the troops further (e.g., not 6 knights per dragon, but 10 or more), it would be logical to leave griffins where they are and move the basic troops, i.e., perceptually, a knight is clearly not higher than a griffin.