There are a couple of threads about the numbering, it's so the same row is the same in each plane. Bulkheads start with this and exit rows with that.

As to him getting on, I understand how he may have ended up with a boarding pass, they could have swapped after they assigned him hie seat, but the scanner should have beeped and stopped him so they could reassign the seat.

It's silly to let people on with a seat that's not even there.