Why was level of the hero downgrade after taking off from the altar?

The altar is a magical place, when you put your hero on the altar, it will directly raise your hero level to the level of the top 6 heroes. When the hero reaches level 240, a new function will appear on the altar, which is to break through the hero's highest level and continue to upgrade. The level of heroes blessed by the altar is green, and once you remove the hero from the altar, the hero loses the altar blessing and returns to his original level. At the same time, the hero level loses its green color.