I dont really get it. For example my sod was 10% Kentucky bluegrass and 90% dwarf fescue. Where is this Kentuckian in my lawn? I dont hear him playing appalachian music. I dont see him buck dancing amongst the fescue dwarves.Whats is the reasoning of mixing? Do they cross fertilize or something to prevent incest. I understand in some cases due to climate you'll have one variety that goes dormant and another will fill in. that makes sense to me. I see that in some mixes, for example an eco fescue seed has some seven seven varieties, why would this be advantageous? The stronger would tend to survive and you end up with what works best in your situation? one could create a nuance green pastel of colors be seperating the seeds and making patterns? Since my overseeding question is different and different than those I've read in other areas, I'll post another topic.
Your guess is quite accurate: whichever is best for your environment is the one that survives.
KBG is a common grass for lawns. Probably the most common.
I don't know anything about dwarf fescue.
I prefer tall fescues.
Annual ryegrass is a bit tricky in a mix. Some say it is a "nurse grass" but I think I would rather just have straight tall fescue. Tall fescue takes a while to get started and annual ryegrass gets real big real fast. So my worry is that the annual ryegrass will outcompete the tall fescue.