Disclaimer: I don't know the numbers involved here at all, but I do understand the concept, so I'm making them up to try to explain this -- the ones I'm using will almost certainly be very wrong.

Let's say your chance to learn parry tanking a mob your level is X. Before there was a bonus for higher level mobs, so maybe for a mob one level higher it's an X+1 chance, two levels higher it's an X+3 chance, or whatever. At some point there's a cap on how good that gets... maybe a mob eight levels higher is an X+15 chance, but anything more than eight levels higher is also X+15.

Valg raised the cap somewhat, so now there's an even higher bonus for some range of levels past where it capped before. To keep using those numbers, maybe now nine levels higher is X+16, ten levels higher is X+18, whatever.

So basically this affects anyone fighting a mob higher level than whatever the cap difference was before, which depending on where that falls could just be hooded mobs, but might not be.