@press-start-gaming-electronics,
With sponsored listings, sellers are paying an extra fee to get extra exposure. Therefore, they are included in search results whether they meet the terms of the search or not. They do try to make sure that the sponsored listings are at least somewhat related, in your example they are games, even if they don't include the exact keyword you requested: "halo". This will happen whether you include descriptions in the search or not.
eBay search is not boolean, and hasn't been for years, since they invented their own "Best Match" search. This is a proprietary search algorithm, which they do not provide details for. Ostensibly, to prevent sellers from gaming the system and unfairly gaining search exposure.
I'm not saying that I like this practice, or that I agree with it. But you asked if it was "expected behavior", and yes, it is.
FYI, if you change the sort order to something like lowest price, or nearest first, you will still see sponsored listings among the search, even if they don't fit the sort order you requested. I don't like that either, but that's how it is.
One way to try to defeat (or limit) the search expansion is to exclude a random keyword, for example < -octopus > in your search. This may cut down on some irrelevent results, but it probably won't exclude the sponsored listings.
