When villagers gain enough experience to level up multiple times (which is possible using certain trades), they'll only ascend one level. This makes it possible for villagers to stay at a certain level despite having a full experience bar, requiring further trading to update the villager to its proper level.

To reproduce, employ a villager to a stonecutter, unlock the mason to apprentice level, gather multiple stacks of stone, and sell the stone until the trade is maximized. Notice that even though the villager is only a journeyman, it has enough experience to be even higher level.