In XY or later, if a Pokemon evolves by a specific level-up threshold and is already at or above that threshold but remains unevolved, either by cancelling evolution, being naturally caught above its evolution level but in its unevolved stage, or by leveling up through a method that skips triggering evolution (such as day care), it gets 20% extra experience, and this bonus is sufficient to trigger the "boosted XP" message even if there isn't cause for one of the larger bonuses such as being traded or holding Lucky Egg.
In Zubat's case, it will start getting this bonus if it reaches level 22 or higher without evolving, but because Golbat is a happiness evolution rather than being pinned to a specific level, Golbat will never be eligible for it regardless of what its level or happiness are.
