Maybe it is all about the safety presentation given just before departure. You know the statement the staff read out before departure "all aircraft are subtly different....you almost certainly sit in a different seat each flight.... so listen carefully......"
Maybe when we don't listen, our lack of attention is noted by secret cameras with highly advanced software that recognizes our body language and lack of atention, so our boarding passes that are partly pertruding from our top pockets are SUCCESSFULLY SCANNED by those same hidden cameras (regardles of OLCI or original QF paper) , and the auto-seat allocation system is instantly updated with our current seat number, so that we are placed there for several of the next flights, until the software in the secret camera notices that we pay attention on a subsequent flight (for at least a few moments) and then resets our seating back to how it was.
Just a thought.....