It may be because of the whole Wembley Season Ticket thingy, where speccies have paid a wedge to have a seat at every live sporting event?

Most of these will be southern footy fans so explains why a few empty picket (mostly in middle tier)

Yes that really p$ssed me off seeing the empty seats in the lower tiers when i couldn't get any the day after beating Catalans but eitherway a great day to be a supporter of the Wire.Probably the best in my 30yrs of being a fan.

There was what looked like a large block of empty seats near us in block 107 when I watched it back on the tv but in truth these were reserved for the choir/dancers who kept coming and going throughout the game.

It's because of the silly system the RFL employ by giving out tickets to clubs to sell then they don't sell them. See the Virtual terrace.

It is the silly policy of having a day when you can buy tickets for a tenner - wait for their face value to go online and then try and sell them for a profit.
I know a bloke who bought 10 tickets at £10 each - ended up going on sale at £41 each - he offered them at £35, sold five and is very happy - result five empty seats - and we wonder why their are ticket touts!