I wish I could figure that out, too!
I have a $100 pair of cycling glasses and a $15 pair from the outdoors store. Both seem the same to me. Both fit fine and I don't get air in my eyes, but I experience the same things as you. I chock it up to my contacts. My eyes seem to get irritated easily anyway. I bought some contact hyper tear drops (as expensive as the cheap glasses) and they work OK. I think it's just the heat, the random crap, the sweat, etc. (for me anyway). I think the only thing that would make it better is wearing ski goggles when riding- but then I'd look like more of a dork than I already do.