This is well-explained in the jQuery comments:

IE adds the HTML element's border, by default it is medium which is 2px

IE 6 and 7 quirks mode the border width is overwritable by the following css html { border: 0; }

IE 7 standards mode, the border is always 2px

This border/offset is typically represented by the clientLeft and clientTop properties

However, in IE6 and 7 quirks mode the clientLeft and clientTop properties are not updated when overwriting it via CSS

Therefore this method will be off by 2px in IE while in quirksmode

The fix is easy though, it's something like this:

if ($.browser.msie) {

var version = parseInt($.browser.version);

if (version >= 6 && version <= 8 && document.compatMode == "BackCompat") {

offset.left -= 2;

offset.top -= 2;

}

}

Since offset() is a mega-important function used for all kinds of popovers and things, it's worth adding a little code to make it reliably accurate.