Legal Anabolic Steroids

Can someone explain the difference between how the legitimate producers make their drugs (specifically AAS) and how the underground labs work?

There is one major difference and it's called STERILITY. There is no way you can completely avoid pollution in home-brewing. Now, if you prepare small doses for your own use you tend to be uber-cautious. Not so if you are a crook in Turkey who wants to make big money fast.

One difference is in sterility, as Bernie7 wrote. Another one is in the ability to properly mix the active substance. This is actually the main danger of counterfeit drugs. And here I mean real drugs containing the proper active substance, not fakes with only sacharose.
This is what people often experience with fake impotence pills: one pill can contain 5 times more substance, another pill 5 times less. This is what happens with poor equipment.

Then of course is the huge problem of lack of control. We've seen UGL's producing one good batch (high level of hormones) and another bad batch (sometimes with even no hormonal substance at all). Some UGL's are actually contractors: they produce whatever you order. You pay more, they put more testosterone, you pay less, they put less of it..

I've seen people brewing gear here in Russia. If they didn't get their testosterone from China that particular month, they just didn't use any. More than that. Almost every "drug" they produced was based on testo enanthate, sometimes cut with prop. This is the cheapest alternative and usually doesn't crash. So whatevere the label (or price), one was basically using the same hormone.

The quality of anabolic steroids on black/grey market has deteriorated in the last 10 years. I've seen steroid powder from China cut with plastic polymer...imagine you inject this.