Looking at flying Christchurch, NZ to Detroit DTW in august (made this flight before).

Virgin Australia will fly me from Christchurch to LA via Brisbane then put me on a code-share with delta.

Just out of interest I hoped on the delta website to look at how much they would charge for their part of the journey. To my surprise I could book the exact same flight from LA to Detroit for considerably cheaper saving myself a few hundred dollars in the process.

This didn't seem right.
Whats going on here VA?