Mathematically, if Apple achieves, say, 10% market share next year, then the 50% figure (for computers in use) would be attained if, say, 50% of the Mac installed user base can play AAA games, but only 6% of the PC user base can.
Making this more concrete: Let's suppose there are a billion computers worldwide, consisting of 900M PC's and 100M Macs. Then the 50% figure would be achieved if 50M Macs and 50M PC's could play AAA games, i.e., if AAA gaming capability were found with 50% of Macs, but only 6% of PC's.
However, the thread's title notwithstanding, I don't think the OP was referring to the % of the installed user base, but rather the % of new computers sold:
[I can't vouch for any of the above figures; I can only vouch for my algebra
.]