You may see at your screenshots under the level number there's an arrow that looks down - it means that you're on the territory where levelling system is in action. And it means that not only your level is down but also stats, hp, mastery points.
Leveling system is working on the locations too.
In some cases hero's level can be lower due to the levelling system. For example, now in Dungeons the level of all characters will be changed to the minimum among the members of a group. If you’re entering a dungeon without a group, the game will try to match heroes with a minimum level scatter — no more than 3.