As the others have mentioned, your ingame Mount collection shows ALL mounts on the account, even the ones you can’t use like opposite faction mounts for example.
Some faction mounts will also unlock the opposite factions version, which is how you can end up with both, Steeds/Wolves, Rams/Raptors, Mechanostriders/Kodos for example.
Mechano-Hog/Mekgineer’s Chopper is another prime example, get one, get the other faction version too.
Near the top of this list is the Armored Snowy Gryphon, which would have come from you learning Armored Blue Wind Rider, just as an example. There’s many more in there too that you can see are Alliance mounts, Stallions, Mechanostriders, Rams etc.
That is the difference between the mount collection number and the achievement though, the difference between account total across both factions and the max usable by a single character on the account.