It's a ratio thing. In other words, you can have 80 degrees out of one side, or 60 degrees on one side with 20 degrees on the other side, or 40 degrees on both sides.
You can believe that, or you can look at the other two or three active heater control problem threads we have going right now.
Essentially, one of your blend doors isn't working. The plastic gear may be stripped, the door may be jammed, or the motor may be bad. Do a search on blend door to find the other recent posts to learn how to pull the codes and repair your problem.