Their take is interesting. They say that people are moving to superior PCs (Apple).
They seem to correlate Windows sales directly with hardware, which isn't totally the case (moving from inferior PCs to superior PCs). That's talking about hardware, while Microsoft does not compete on a hardware level in the PC market.
I highly doubt that for every percentage drop in Windows usage you'll see a percentage increase in OS X usage.
Seems like they are struggling to find something newsworthy here.
EDIT: According to the article's link, OS X gained 2.05% and Windows lost 3.21% from 7/08 to 5/09, confirming people aren't always moving from Windows to OS X.
It's also interesting the chart shows iPhone and the iPod Touch too. I know of noone who has ditched Windows to move entirely to an iPod Touch.