It's dependent on how you got to where you were, for the most part. Developers are heavily suggested to build a "backstack" if the expected usage is to go back up through a menu instead of returning to the last activity.
For example, if you read an email, then load up a map through a link. Which behavior should take precedence when pressing back? Going back up through the maps hierarchy or going back to your email you just came from?
The ambiguity has led to the creation of the action bar and the "secondary" back button on the top left. Now in 3.x +, if you want to drill up in the application you use the action bar, if you want to switch back in the order you came to the activity you press back (unless the developer creating a custom backstack that drills you up through the application anyway lol).