*Note* In terms of weapons, it's not all weapons, only some. My PC is in the shop right now so I can't pull up which weapon it was I saw this on right now.
Interesting. Generally when I add mods to say weapons, the stats will change. For example, I had level 4 iron chest armour and added the muffle mod, the mod says it reduces noise by 4%, but the stat did not change. Same with mods that increase mobility, or plate mods don't show an increase of armour rating.
As for the horse, 21 to 18 seconds isn't huge, so that's probably why I didn't notice. No biggie, I didn't do any actual testing on that.