Basically the coverage of the panels is lesser. Meaning if you have some pixels here and there they will not allow you to request for an exchange.
Perfect pixel coverage is much better, you can read what they cover.
Pixel perfect used to be really perfect..
Nowadays many sellers, even dream seller use "Perfect Pixel" but if you read properly, most of them don't cover fully.
Many of these sellers state that they only consider "Stuck Pixels" as imperfect, and allow up to 3 stuck pixels for their so called "Perfect" panels.
So technically you can have 2 bright stuck pixels and 10 dead pixels and they'll call it perfect
Advise to read carefully on their TnCs of Perfect Pixel editions before purchase. I think sellers like AccessoriesWhole still sell true perfect panels.