Problem is, the pre-made group finder shows their total item level, not their equipped. So in total they could be 430, but their equipped item ilvl is only 420.
There are positives and negatives for that kind of system however.
You could have a lower ilvl because some pieces of gear you got on (such as rings and trinkets) are better than any higher ilvl alternatives. I should know, the tank trinket from Opulence is still a very strong trinket for tanks.
However, they can inflate their ilvl by having poorly itemized pieces of gear in their bags. Thereby somewhat “exploiting” the system by presenting a false ilvl.
Sometimes you have better itemized gear for your offspecs than your main specs. it can happen. my main hand fury weapon is 435 ilvl, but my prot main hand is still 415. (offhand for fury is 420 with socket, and my shield is 435).
Purely looking at item level alone can be dangerous. Ever hear of the “great gear, poor player” phrase?