css.('font-size') in IE

css.('font-size') in IE

This has me baffled. In FF, Opera and Safari $(this).css('font-size') gives me the actual font-size in px. But in IE7 I get a px-value I cannot decipher.
I declare the font-size in my style sheet in ems, and I get the strangest results. Sometimes it is the font-size but multiplied again with the ems (so if font-size is 2em (28px) in all browsers but IE states it is 28*2=56px. But sometimes I get values that run into the 100s which have no relation to anything at all as far as I can see.
Despite this, IE does display the font-size correctly. So what does IE actually pass unto jQuery as the font-size?