Kerning different on Mac and PC

Adobe garamond pro (otf) (and perhaps other opentype fonts) renders differently using MS Word 2010 (PC) vs MS Word 2011 (for Mac).  Everything is set the same-- kerning, font size, margins, etc.  And the font version is the same.  But invariably the characters are placed more tightly together within Word 2010 and spaced a bit farther apart using Word 2011.  It's not some optical illusion:  Word 2010 fits more characters per line (not many more, but there's a difference).  The Word 2010 version looks better.  Also, for whatever reason the difference is accentuated when converting the Word 2011 document to a pdf.  The document converts perfectly from Word 2010.  So, my question is:

What accounts for the difference and is there a way to fix it with Word 2011?  Why would the same font render differently if otf is a universal platform?

Thanks.