In Firefox 3.5 they both look exactly the same:
$('topics topic',lessons) == 4
In IE6, it differs, with:
$('topics topic',lessons) == 48
I think this is related to an issue where with IE and jQuery it is not
reading the returned data as XML because the response type is not text/
xml. If you do a search on it you'll find some solutions. The most
common solution is to force the response type of the returned XML
(tos.xml) to be text/xml, if possible.