The numbers reflect recent sales, not the number of users. If I remember right, Mac users tend to use their computers longer then PC users, so there's an accumulative effect of users of Macs. That HP had a 30% failure rate within the first 3 years also cleaned out a lot of older non-Macs from the heard.
I'm not sure if this next stat is correct any longer, but at one time I remember a report that 30% of on-line surfing was done with Macs.
I know if I go into most coffee shops, Macs are far more common than all the other brands combined. Most of them are being used by students, so if that is any measure of preference, younger adults are choosing portable Macs over non-Macs by somewhere around +50% of the time.
This leaves the stodgy business buyers that are picking up the slack with desktop PCs to skew the numbers to 90%.