I think the 'Linux' numbers are basically flat. The variations are just statistical noise depending mostly on whether or not Ubuntu had a bad upgrade cycle (i.e. your display stops working-- a notorious problem with Linux upgrades) for the year in question.
Anyway, 'Linux' means squat to most people (even Windows users who follow Apple) because there is no firmly identifiable interface for the user (or the applications programmer, for that matter). Only interfaces that system programmers (Linux hackers) use get any respect, while GUIs need to be able to disappear in a millisecond when hitting Ctrl-Alt-Bksp. Imagine if Linux selectively and instantly killed the entire contents of /dev by pressing three keys... but that would never become a standard feature because it would be unthinkable to Linus' clique of hacks. People don't care about 'distros', they care about their chosen platform, so its no wonder most people wouldn't know a "Linux" if it hit them.
#16 has a good point, except that Linux cannot be considered a platform in this context. It's a pile of parts, with no set of pieces fitted more slipshod than the GUI elements. That is desktop death.
Apple is Killing Linux on the Desktop