Scrolling: 2012 MacBook Pro vs. 2012 MacBook Air

So, I was at the Apple Store today looking at the MacBook Air and the MacBook Pro because I was thinking about buying one, but wasn't sure which one I should buy. I remembered that when I had my 2010 MacBook Air, it'd always lag when scrolling on different websites like The Verge, Engadget, FaceBook, etc., while two of my friends had their MacBook Pro's on the same websites and theirs wouldn't lag at all when they scrolled the exact same websites right next to me.

I did the same test today on the new 2012 MacBook Pro's and Air's and the Pro's didn't lag at all on any of the websites, but the MacBook Air's did. So, now I'm wondering what is the cause of the jittery scrolling on the MacBook Air's? I figured that on my 2010 model, it was caused by the slower Core 2 Duo processors, but the 2012 MacBook Air's have the same issue and they're running on the new Ivy Bridge processors.

So, does anyone know what seems to be causing this issue? It's always confused me.

Note: This is comparing the 2012 13" MacBook Pro and the 2012 13" MacBook Air.