But they're not... The Google Music, Reader, Market and others that I can't think of use the top left icon and a left handed arrow to take you back in the apps activity and that works regardless of the actual Activity stack (And the back button works as a stack navigator).

This inconsistency and change in Google apps' behaviors is frustrating and is the only thing that I think they failed at in their attempts to fix the "consistency" problems of Android UI/UX.

I'm with the grandparent, I'd like to see this clarified. Some picture-based examples might help explain this issue better to those unfamiliar with Activity management.