IEEE may be more prone to precision errors (letting bad papers in) while NIPS may be prone to recall errors (throwing good papers out).
With the way reviewing is done (no one can take a week off to read and fully comprehend the four papers they are given) you cannot achieve perfect separation - even if that were possible.
Calling the process of "accepting a SciGen-generated paper into a allegedly peer-reviewed journal" a "precision error" is a bit on the optimistic side. It implies that someone was making a decision after reading the content of the paper, as opposed to, well, just accepting everything in sight.
It doesn't take a "week off" to notice that a paper is gibberish, at the very least.