I made the same point elsewhere, but I guess the logic is pretty simple.
If you buy a ticket for a one-off game, you almost certainly go to the game. Attendance of these folk must be 99%+.
Season ticket holders make around 90% of games on average, I think.
Subtle but nice functionality enhancement there to show the blocks that are sold out like that. Only a couple of days ago it still wasn't doing that and a user had to go to each block, one by one, to check availability.