Just to further put this idea in the dirt, see the above pic. Not that I'm pointing fingers or anything (okay, I guess I am), but AAS use doesn't seem to have had wacky effects on Nasser El Sonbaty's massively prominent and evenly aligned 4-pack.
It doesn't make any more sense than the first theory. If this were true, then doing dumbbell curls would put you at risk of developing drastically mismatched biceps (which is generally not the case.)
All the previous posts have nailed it. It's genetically determined. Build some muscle, lose some fat, and see what you end up with.