When a string has tension pulled it stretches. When clamping there is usually some tension lost because of drawback, friction, poor clamping, clamps too loose that slip etc. this results in a string bed tension lower than the strung tension.
Sometimes when using fixed clamps there is some play in the clamps and some stringers like to pre-load the clamps by pushing or pulling on them. This action can actually increase tension in the racket. Here is an example where you can see what I mean.
When you tension a string you stretch it. If you have a constant pull (drop weight is constant pull) the longer you pull the more you stretch. The longer you stretch the higher the tension in the racket although the tension pulled never changed.
So back to my original question have you checked the pulling tension of your stringer?