The thing is people did move about. Quite often seats would show as sold one week and then the next week they were available again as the person or people who bought the ticket, then moved elsewhere.
So when you were shown the screen it may have been correct, but those 3 next to you may have moved elsewhere, when they found out that say friends where seated somewhere else an had 3 seats next to them.
I know that 2 of our family have moved from the south stand, so there should be 2 seats together going spare there, if you get a single infront or behind it's pretty much the same as 3 in a row.