I guess this is the same problem as other sites have, but for the sake of correctness, the following is only valid for Steam; for other platforms, this does not apply.
The public games list just lists appids, which is mostly true, but does not really match up that Steam not licenses "apps"(id) directly, but packages (= subs) which have their own set of unrelated IDs (subs id).
For an example, I use the Alone in the Dark Anthology, which contains Alone in the Dark 1,2,3 and "Alone in the Dark" (= the bad step brother nobody talks about it, but wants to usurp the family name aka part 4).
This is https://store.steampowered.com/sub/135448/ (note that the URL speaks of "sub", not "appid").
SteamDB representation: https://steamdb.info/sub/135448/apps/
According to gg.deals I only own "Alone in the Dark" from it, but not the Anthology, nor 1,2 or 3.
In the global search the situation is like here described (green = owned, red = I should have, orange = game package, should have too).
This is explained if you look at my public Steam games list, where it also only lists both games gg.deals is aware of:
As you can see, there is no mention of the other games, although I own them. A similiar situation is that all these bundles and editions are also "just" packages which do not have a (matching) appid, but a sub id which is not known. Another possibility is that the same game was assigned a new appid altogether which may occur if a game licenses gets a new publisher. The old license hold decides to delist the game and the new publisher has to submit it again and therefore, the game gets a new appid.
How to fix this? There is a JSON object for every Steam user available, located on https://store.steampowered.com/dynamicstore/userdata (you have to be logged into the Steam website for this, afaik, dunno if the current authentication method is enough). This JSON contains - alongside other things - a full list of all subs and appids which the user has licensed.
If we want a more complete and thorough representation here on gg.deals, the package ids (subs) should be integrated.