Tunelab question

[Michael Gebhardt]

Hello everyone!

I've tried using Tunelab as a teaching tool and noticed, 
that the phase display for a note that is close to pitch, 
first moves to the right for a few seconds after striking 
the key, gets slower, and then moves to the left.

Is this "pitch change" an actual phenomenon in pianos or an 
artifact of sampling in the computer? If it is the former, 
what should you use for tuning? The stabilized display, I 
guess, since this is what you would use with aural 
listening, right?

Greetings

Michael