I know real natties who have these too. They can be natural for the most part, but I'm sure that some of them are related to drugs. I've also heard of certain steroids causing them to protrude more than normal.
I'm lean enough to see a couple of these on my stomach as well. You can hardly feel them when there's adipose covering them, but lo and behold. Never injected anything in those areas too so it's not from that.