The numbers we get on the mac are often 10-20px smaller than the numbers we get on Windows. This matters because it is one of the factors that makes our text inputs smaller than IE on Mac WebKit.
Currently we grab the maximumAdvancement off the NSFont. On Chromium Linux (which gets the same values as Windows for Windows fonts), we compute the maxCharWidth as the xMax-xMin of the font by looking at the hmtx font table. Or something like that. :) Amanda is working on a patch that tries to do the same on the Mac.