I'm sure it doesn't account for it all but...
It strikes me that the market share in XP can fall as a percentage without falling as an absolute number simply by an increase in the number of non-XP systems. Some back-of-the-napkin math:
To start you have:
10 XP,10 W7, 10 W8
That's 33% market share for XP.
Then increase the others:
10 XP, 20 W7, 20 W8
XP now only has 20% market share but nobody that was using XP has actually stopped using XP.