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?
If you click on the 'Day' view and then 'Show only Mavericks' you can clearly see a rise from 0.20% at 7pm yesterday up to almost 10% at 5pm today. The other lines have had a slight decrease, but yes it does look like Mountain Lion users are upgrading fastest. I'd be surprised if Leopard/Tiger users upgraded quickly as I'm sure they'd be apprehensive of updating their old and potentially fragile kit.