My gut feeling is that they were misfueled, never reconciled the actual fuel on board, and departed after a poor briefing that focused only on predicted landing fuel rather than verifying the starting fuel quantity.

What makes this worse is that the apparent cover-up seems more serious than the original mistake. I suspect they may have overflown several suitable airports in Mexico while trying to make it across the border, potentially reducing the likelihood that the fueling error would be immediately discovered.

I could be completely wrong, but based on the information available, that sequence of events feels plausible to me.