A number of the set checklists on here have an added card or two that are not part of the set, but count towards the percentage of the set you have.
As an example I just got a box of 2002 Topps Hockey of which I got 211 of the base set cards. But because there was a redemption card in the box and someone added the redemption card as part of the base set instead of an insert the site says I have 212 cards. Somtimes it is a redemption card, sometimes an ad that is randomly inserted and sometimes it is the checklists for that set. I just think they should be inserts and not part of the set.
Rant over.
