Yeah that's odd. Possibly related: note that planting an outpost on a swamp tile, or raising and then lowering land on it, will convert a 'swamp' tile to a 'plain'. Swamps can never be settled, plains sometimes can, so sometimes* you can legitimately open up extra fertile tiles this way, if the underlying plain should have been fertile but the swamp was blocking it.
This is easy to replicate if you have the spell that revives land - cast it on a swamp, note that the swamp is still infertile. Now raise/lower land and see how it sometimes* opens up fertile plains tiles. The way an outpost converts the tile underneath it to plains functions similarly - although this only applies to the actual tile under the outpost building, and it doesn't become settleable until you raze the outpost. Random other tiles within the outpost's ZOC becoming fertile is definitely a bug.
*Tested and confirmed that this (sometimes) works. Very inconsistent, but I have found some swamp tiles that do suddenly become fertile after a raise land, or founding and then razing an outpost on them.
