It says 'per paying spectator' so presumably includes STH, which for us make up a massive proportion of the total gate - and if you take into account concessionary STHs, the average price of all STHs is maybe around £15 so once you add the rest I can see where an average of £20 comes from ?
But that raises new issues. As well as the idea that to get an average of 20.68 per game you need to include tickets that aren't actually available to a full paying adult (i.e. children's season tickets), even with season tickets included (the cheapest adult season ticket works out at 20.22 per game just below the average), but you now have the idea that this is being put out there with the intimation that it's the "cost of going to football" with the average ticket price one would expect to pay per game. An adult cannot pay £20.22 for one game. The only way they can get that price is paying for all games, and suddenly a theoretical outlay of £20.22 becomes a minimum actual spend of £465. It's like those cookery shows that say you can make this meal for only £2.50 per serving, ignoring that you can't just buy half a teaspoon of paprika, a tablespoon of mayo, half an onion, etc.
I wouldn't mind them promoting that as the average, but they should also include real term costs, average match day cost for each age bracket, etc.