Going off the 'chart' (which is probably the worst chart I've ever seen), it looks to me like the bottom line has dropped a lot. I think the bottom line probably isn't Mountain Lion?

Are you using some chart where you can actually see which OS is which ?

EDIT: The chart is broken in Firefox - it works fine in Webkit only. Goodbye open web, was nice knowing you.

Anyway, I see you're right. Lion and Snow Leopard are both roughly where they started. So I change my opinion - the 7-8% who have upgraded to Mavericks are likely the people who follow Apple closely and so were already on Mountain Lion to start with. This might signal that the fast upgrade speed may slow down quickly as it implies only the small part of the OSX population who follows Apple news is upgrading so far?