Mounts displaying incorrect spell duration

[Night Hawk]

I've been experimenting with a hud timer lately and I noticed the other day a spell that usually shows 6 minutes showed as 3 minutes, and it had me confused for a while. I finally noticed it only showed it incorrectly when I casted it from a mount. Problem is it doesn't do it to all spells, and I've only found 1 or 2 spells that I use so far.