Either you don't recognize it as a part, or have not cataloged?
I have quite a few, and for different reasons. Sometimes if I find an incomplete game, say for instance, when I found my $1 copy of Silent Hill disc only. Great game, but, until has a case I pretend it doesn't exist.
In other cases, there's some obscure stuff I never cataloged because I just didn't want to go digging it all out, like my Atari, Coleco and Vectex stuff. So even though I consider that stuff in my collection, it's not cataloged.
Other times there are just games I wish I had never bought, shovelware crap I accumulated when I first started, that I purposely leave out. Banished from the collection!
Then there's just stuff I forgot I had, and never added it to my page - not too long ago, I remembered I had found a couple of CIB SNES games back in 2004ish, before I got big into collecting - one of which, was none other than Castlevania IV. Still haven't added that one!
Then there's some games I trashed picked and haven't decided if I want them, or not.