In fact, the real Adelaide was captured by her own doppelgänger as a little girl and trapped in underground tunnels with the rest of the tethered. The doppelgänger then lives her fantasy of a life above ground, and we discover that the woman we've been rooting for all along was, in actuality, Adelaide's supposedly evil tether.
This is very complicated to explain, but if you've seen the movie, you'll understand, and if you haven't — well, I told you there'd be spoilers.
Where does Jason come into this, you ask? Well.