I think it's just a natural reaction when you're there with a girl to think she's better looking.
You're there looking through a bunch of pictures on AW, you've seen a hundred girls that day already, you may well have been looking at porn too, where the girls are hotter than most of the prossies you've been looking through. It's pretty hard to be particularly impressed.
Then you turn up to an actual meeting. The girl is right there in front of you in the flesh, half-naked, all her attention (hopefully) focused on you. I think it's natural to be more impressed by the girl at this point than when you were casually browsing through her gallery.
Maybe that's just me, but I think that's the reason why the cliché of "looks better than her pictures" almost always applies when I meet a girl - with the obvious exception of when the pictures turn out to have been from 10 years ago, or of a totally different girl.