If the game sells out just before the game statrts, it means that the pricing charged was the correct price to maximise revenue. Sell out too early, and it means that you could have charged more....
Given the way the tickets are being released for sale, it's unsurprising that tickets are still...
If they were only charging £1 for this match against Arsenal, it still wouldn't have sold out yet as all tickets won't have been on sale yet.
Just because they are charging more for this game doesn't mean that they will slow down their release of tickets for sale to fans using the same method...