It's because of the way dungeon loot works. If I'm an 840 druid this week, there's very little advantage for me to run mythics with non-leatherwearing 820 anything if I want to be as geared as I can for raid opening week.

I do 845+ mythic groups, because there's literally only a few things that drop that can upgrade my gear anymore. I need a ring and legs with better tank stats on them, and of course a legendary, and that's about it. If I take an 820, what are the odds that if legs drop that he will be able to trade it to me? Probably next to none, even if he wanted to, because his ilvl in that slot is too low for him to trade it. If I go with a bunch of 845+ leather classes, all of a sudden, the chances of me getting my leg slot upgraded is much higher.

People are listing 845+ ilvl in group finder not because they want to swing their ePenis around to show how high their gear score it, but its really because it's the easiest way for them to get gear.

I'll list a mythic pug group at 845+, but for leather wearers, I'll take anything down to high 830ish, just because it's pretty much the higheset chance I'll be able to get the upgrades I need.