[Hoff]

Looks to be browser fragmentation.

"Other" (which is massively increased) and IE9 are the only increasing entries in this plot:

http://gs.statcounter.com/#browser_version-ww-monthly-200809...

While what's in that "other" plot isn't listed, the Chrome and Firefox version schemes and their associated plethora of smaller and more frequent releases have played havoc with the classic version-plot scheme.

[ootachi]

I don't think it's fragmentation. It's Chrome on track to achieve market dominance.