Thanks guys.
My mistake: I'm talking about a K-r.
The settings available for the E-dial in P mode are:
P Shift
Tv
Av
OFF
I'll try to give an example so it'll be more clear.
If I set the E-dial to P Shift and press halfway the shutter release I may get the following settings (I will consider a fixed ISO to eliminate one variable):
f5/200
If I move the E-dial one notch to the right I will get f5.6/160
The camera closes the aperture to decrease the exposure with 1/3 EV and slows the shutter down to compensate and regain the third of a stop.
Press the GB - all back to the initial settings.
If I set the E-dial to Av and make the same steps, the same thing will happen.
I will have first f5/200, move the E-dial to the right and get f5.6/160.
GB will reset the values.
In other words, for any setting of the E-dial in P mode, the camera will adjust the aperture/shutter speed pair in order to keep the measured exposure.
And I'm ready to bet the same will happen if I will set it to Tv.
So why 3 settings available?
What other things are changing that I'm not observing when using Av instead of P Shift?