Vista - Browser-like app creates new pages on Navigation?

I just figured out, in a browser-like WPF application, whenever I use
navigation to go back or forward in the nav history (journal), my page
objects are not reused, but created anew instead.

My question here is: where does it store the data?

For example, I've got a Textbox and a Listbox on my Page.

So, I navigate to the page, edit the Textbox, fill some items into the
Listbox (using controls on said page). I navigate to some other Page
(or PageFunction), then back again.

Since the navigation does create the Page anew the Listbox is empty
now, again.

But the Textbox still contains the text I just entered.

Strange. I'd like to understand: how/where is this stuff stored? Where
does the content go (and come back from again?)

Thanks,
Sam