Evolution of the AI.

Yes, if you look at this picture, the best Black Dragon is in the fourth game (strong and well-fed). But in the fifth and sixth games, the Black Dragons are more like spectral dragons, which necromancers starved and then resurrected. In the second and third games, the Black Dragons are also quite good, and in Might and Magic VIII, they don't evoke pity. But in the fifth and especially the sixth game...