SpacemanSpiffed: I have a number of games where I purchased the base game, then purchased an Upgrade DLC for it (Soundtrack, Artbook, etc) so I have all the same content as the "Deluxe Version" even though I don't specifically have the "Deluxe Version" SKU. So when the Deluxe Version appears in a sale or the main page, it is not marked as "In Library" and I sometimes click through only to waste my time realizing that I do have it.
This unfortunately also works the other way around.
I have Deluxe versions which don't show the seperate DLC as owned on the store page.
For instance I own Metro Exodus Gold Edition (includes the DLC), but the Expansion Pass isn't marked as owned. The same goes for The Witcher 3, Pillars of Eternity: The White March, etc. It's not just story DLC, it's also sountracks, etc. which have this issue.