This is intentional. Windows uses font hinting which attempts to align the anti-aliased font to whole pixels vertically and sub-pixels horizontally. This is done to maintain readability at small sizes. Apple doesn't use this type of font hinting and applies sub-pixel anti-aliasing proportionately.

See this comparison of the two methods in detail: http://damieng.com/blog/2007/06/13/font-rendering-philosophi...