Fonts in Ubuntu can be absolutely unpredictable when it comes rendering time. I remember an install of msttcore-fonts that I thought would allow me to see sites built with Verdana, Tahoma, etc. similarly to Chrome on a windows box. Not even close, and I'm talking about the kerning, not even the subpixel quality.

IIRC as sibsib said windows takes major liberties with the kerning to keep its core fonts on the pixel grid, and if msttcore-fonts are hinted, they sure don't come out at all similar to the windows versions.