One thing I notice quite a bit on some flights are people when they board will show the FA their BP and then FA will audibly tell them the seat number and tell them approximately where to go.
My first assumption is inexperienced flyers, but then I also realized that anywhere in the world, there are people who are, to one extent or another, illiterate and cannot read the bp... or perhaps can't make the connection between the number on the boarding pass and the seat number., or are n a challenging situation due to language differences. Also, I have to confess I have boarded aircraft where the seat numbers are ambiguous (aisle vs window), or they are in some weird place like on the armrest or on the video screen where someone boarding in a hurry just won't see it, and I have taken the wrong seat.

So, I always assume innocence until there is evidence to suggest otherwise, and simply smile, showing the person my bp, and an 'excuse me but I think we have some seat confusion here' and see what the response is. It's usually an error and no reason for a confrontation... as a recent deadheading TK pilot nearly created with me when he told me to get out of his seat and demanded to see my boarding pass before realizing he was in 2A and not in 3A.