What I am saying is that if I click the email app and open it, then I click an email read it and want to go back to my inbox the back button works in a logical way, it takes me back to my inbox.
If however I click a notification that takes me directly to an email and then want to go back to the inbox the back button does not achieve this. This inconsistency is one of the things that bothers me about android. I understand that it could be considered a personal preference thing and I am fine with that, but my preference is for a consistency in the back action which iOS achieves really well with a swipe and android handles inconsistently in my opinion.
Now I understand what you are saying. There is a way to get there, its just not the back button. The back button, from my experience, takes you to the last place you were. If you came in from notifications, then the notifications was a drop down on the home page. Its a system back, not an application back. I actually like having the back option, because there isn't an easy way to go back to a prior app at all in iOS. You have to go to recent apps and select the other app again. No way to just one click back and forth.