Since a pretty recent update in Wrath of the Lich King classic, a retail-like change was made: if you own atleast one 310% (extremely fast) flying mount, any mount you’d use would also be 310% speed.
However, this is bugged. I own the Ironbound Proto-Drake, both the achievement and the mount itself on the same character, and it has 310% flying speed just fine.
But when I use another mount, it still only uses 280% flying speed.
I’ve been testing it and it seems to not just be a visual bug.
Working as it should. The new flightskill was not introduced (that give 310 on all flying mounts). So gryphons and what not have 280 still while the special proto drakes still have 310.