A330-200 vs. 757-200 for same flight

Just a curiosity question.

On Friday night, I'm booked on a flight from LAX to ATL. When I go to the confirmed seat map for the flight, it shows the layout for an A330-200. But if I go to book a new seat on the same flight, the seat map shows the layout for a 757-200.

I am confident that there is a logical reasson why, but that reason is lost on me. Any thoughts?

Michael