This problem occurs when using the Minecraft level formula for levels 17 to 31. So, for example, if you have 20 levels and 0 points, using the following formula ((2.5*level^2)+(40.5*level)+360) you get 550 as a result. So, theoretically, by using the following command /xp add @s 550 points, you should get 20 levels, but actually the executor gets 20 levels-1 point.