There was a 767 that had a maintainance problem the other day resulting in the flight being cancelled. I think the #2 engine would'nt start on pushback.
I guess it's a possibility that this a/c could still be in MEX
waiting for parts, etc.
The second 767 is probably the regular sked flight.
In life, some days you are the bug..... some days you are the windshield!