By any chance, did you pre-order the game? If so, then that would account for it. When the game was released, the original store page was taken down and a new store page was put up for the post-release version of the game. The difference is that the pre-order version of the game includes the Toby's Dream DLC bonus level, which is not available to those who purchased the game after release.
A similar thing happened to me with another game. I purchased "Neverwinter Nights: Enhanced Edition Digital Deluxe Edition" last year before the "Tyrants of the Moonsea" DLC was released. Prior to that DLC being released, the store page for "Neverwinter Nights: Enhanced Edition" (the base game, which is part of the Digital Deluxe Edition package/bundle) showed that I owned it; since that DLC was released, the store page for NWN:EE shows that I do not own it and can buy it. When I asked GOG support why the base game no longer shows as being owned on the store page, the response I received was "it is not shown as owned due to a limitation of our system".
EDIT: It turns out that on 2019-12-10, Frozenbyte made the Toby's Dream DLC available to all owners of the game, so that would be why the store page changed -- it changed from the base game only to a package which includes the base game and the Toby's Dream DLC.
Post edited May 30, 2020 by cmclout