The idea is that dwarves have the highest chance of getting favorable stats for a mage career. I mean, their defense and magic skills mainly increase. And that's very logical for a mage (a mage casts spells, and units take the hit and also hold up well). Plus, the resurrection and incorporeality runes help.

I remember playing as Inga, and as you said, her defense and magic skills grew normally, but with others, I had cases where the stats were approximately like this (this is at the early levels): attack 4, defense 6, magic 4, knowledge 2, and there was an artifact that gave +2 to knowledge... honestly, the dwarves' stat distribution isn't very specific, unlike orcs. I think dwarves need to take education in many cases if we want them to become full-fledged mages:D ;) ...the hero at the start had attack (Ibba), maybe that's why attack accumulated?!