--and this is due to the fact that naturaly, lipoic acid only occurs in the R-isomer form, but commercial preparations have normally contained both the R and S-isomer forms, and the latter is not is a bodily usable form (having to do, chemically, with the "handedness" of the molecules and their ability to fit into cellular reaction mechanisms).
The Wikipedia article explains this, though rather technically. The Linus Pauling Institute article is a bit less technical:
The usual efficacy I've seen quoted is that the amount of usable Lipoic acid from R-lipoic is roughly six to ten times that of a racemic mixture, so if one is taking 600mg of alpha lipoic each day, one can get a similar effect from 60-100mg of R-lipoic. Of course, there are individual differences. I went from 600mg of alpha lipoic to 100mg of R-lipoic, and seem to get roughly the same effect. The other advantage is that some people find the supplement, well, acidy, and the smaller amount of R-lipoic needed tends to help with that.