the setting 601/709 means: no remapping applied.
The setting "RGB" means: remapping applied (luminance stretch to full RGB 0-255).
The same on import: 601/709 = no remapping. RGB = remapping (on import it's luminance compression instead of luminance stretch).
Back to your comparision. Just noticed you are on a Windows Computer... maybe Quicktime behaves completey different on PC (as sRGB is the system's color space). I think that explains the different outcome of our tests.
Correct, the RGB shift was expected for the reasons you mentionned. What is crazy is that we are doing exactly the same settings, with the same monitor gamma but from 2 different OS and there is a difference. Apple in your case shows the shift (I assume you are on Mac) while in my case (PC) it displays correctly, both in QT.
The sRGB does not convinced me that it could give such a difference. Actually, when the shift occurs I loose almost 3 stops. It affects the luminance, everything. That is more than that IMO.
So working between the 2 OS is a mess.