Could be several reasons, most of them lying with PP I think.
Anyway from the cart side, you're using the Website Payment Standards module in 641a (it'll just be called paypal.php and have $Id: paypal.php,v 2.8 2004/09/11 devosc Exp $ in the header of the file) with itemized order details selected in the modules settings right?
PP has limitations on field lengths that it accepts for product information. Some are 64 characters.
Also, given its a free text entry, there may be other security reasons why this type of attribute isn't accepted at the PP end of things, and therefore not returned.
You can read more about the spec here:
https://cms.paypal.com/us/cgi-bin/?cmd= ... lvariablesSimon