We don’t know that.
They could have been tickets from ticket partner sites that didn’t sell (they are often priced at £350 and well above for finals so can see why people from our support wouldn’t buy them).
That area of the bottom tier along that side is where all the players’ families sit at Wembley. When we’ve lost finals, I’ve always stayed after the game and that area is always still busy with people who the players go over to. Maybe our players didn’t move enough along to friends+family.
Maybe there was a fuck up with the City ticket site.
Maybe the FA couldn’t shift them on to the FA family.