It is normal, nothing to worry about. It is the same as normal leveling up in battle; the pokemon base Hp goes up and so does the current HP. The amount of HP recoverd depends on how much the base goes up, and if a pokemon's HP is 0, then if the current amount increases, a pokemon cant have HP when it is fainted, so it is revived. This is a little bonus help from Rare Candys!!! :)