If I remember correctly that's because the dwarf and orc models have been in the works for a lot longer than the others. They originally said that they weren't even sure if they'd get anything apart from dwarf and orc out for launch and that the others would roll out as the expansion progressed. Seeing as how they've decided to release all at launch it makes sense that the newer ones might look a little rushed, and less detailed, compared to the dwarf and orc. Not that this makes me happy, I for one wish they had put off the other ones until quality was the same, even if this means a roll out release.