It seems that when navigating back inside you application (not with tombstoning) the previous page seems to be kept in memory and retains its state. Class fields and properties are still set and don’t need to be reinstantiated. NavigatedTo and Page loaded events are still fired though.
I found this out the hard way when I dynamically added buttons to the ApplicationBar. I just added the buttons to the bar in the NavigatedTo event and wondered why it added more and more buttons whenever I navigated back to the page. It did not happen when I navigated directly to the page. This behaviour does not seem to be documented in the SDK.
So you might check your instance fields bevore recreating them. Might save you some processing time when going back.