That’s how it should be working. Unfortunately, mount speed bonuses in classic are being calculated with the post-WoD method currently used in retail - it’s adding a percentage of your walking speed onto your mounted speed, rather than using your speed while mounted as a base and increasing it by a percentage like it used to in every iteration of the game before Warlords of Draenor.
So if you equip a carrot on a stick with an epic mount, right now in classic it’s only making you go 1.5% faster than you were before you equipped it.
I made a thread about it here if you’re curious: Mount speed is being calculated incorrectly with the post-WoD method