Okay, let's take it a stage further. The attendance given on Saturday was 29,469, that's 1,281 less than the capacity of 30,750. Factor in seats not used due to segregation areas, seats for hospitality guests - WAGS, club employees etc., then there's not going to be much change left over. Hence...
If the club says they sold out, then they sold out. Why on earth would they want to lose out on the revenue? The reason for empty seats is simple - people who had tickets didn't turn up.