For some reason, I thought it was simply going left to right, starting with the left-most mob currently fully visible onscreen. Thus, if my character is standing within arm's length of mob that is to her left, but the mob is half off-screen, tab-targeting will ignore that mob and move to the next mob to the right, regardless of its physical distance to/from my character.

Of course, that's not what I want, but for some reason, I thought that's how it worked.