If your main is a non-class-mount class, then you need to log in your class mount for the achievement counter to reflect it. For example, your main might have 227, your paladin has 230. You learn a mount on your main, your main total goes up to 228, which doesn't up the achievement. You then log on your class-specific alt (paladin, lock, DK), and then your counter will tick up from your alt having the new mount.