[Von Moltke]

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..