Almost 8 pages... and so much misunderstanding.
It's pretty easy.
A "perfect" fifth doesn't beat.
A perfect, non beating fifth's frequency is 3:2 ratio of the root.
A perfect octave is 2:1.
Start doing the math for the circle of 5ths from any arbitrary root tone (I will use A4 at 440 for no particular reason)
440/2 x 3 = E660
660/2 x 3 = B990
990/2 x 3 = G#1485 ect
If you keep going all the way around the circle back to A, you will end up at 57,088.3887
But if you go up by 2:1 octaves and just keep doubling 440 by octaves you eventually get to A11 at 56,320.
I know, I know only a dog or maybe some insects can hear that, but that's not the point. The point is assending by perfect 5ths doesn't quite line up with perfect octaves.
So there you go.
We could tune a keyboard to have "perfect" non beating 5ths but the octaves would be off at either end and you could only play it in one key. So, we have pushed the tuning of the 5th to be a little out of tune; it is "tempered" to allow the octaves to meet up.
This is not "stretch". Stretch is something else that is done in response to the overtones that are never quite in tune with the fundamental.
So, to make a long story short, that $150,000 Steinway D is NOT IN TUNE!