On-ground Pressurization Test

[JamesC]

TC output efficiency will increase (to a point) with altitude (colder air) and since the TC supplies the upper deck pressure and cabin pressure, there may be more air left over for at altitude for keeping the cabin pressure up. Otherwise diff should not change with altitude, but you are far enough from max diff that you can see it. That is my guess.
re 'home' pressurization test - do-able if you have the electric door seal option. Otherwise it is tough for a shop vac to blow enough PSI to inflate the door seal. Some have used one or two leaf blowers and a Halloween fog generator running inside the cabin on an inverter pack.
I agree with Herb - take it to a good shop with a huffer cart - they will find and seal the leaks much more quickly.