It was a finger in the air estimate mate. Whatever the actual number is, they will have worked it out based on what they think they can get us to pay for it given the scarcity of seats. That number would have been lower at a higher capacity and overall it makes more commercial sense to thrm to have the lower capacity.

You also have to factor in how often it will be full. At 53k it's pretty much gonna be every seat sold for the foreseeable, but at 63k of the team is playing bad or economic circumstances dictate then there would be a fair few empty.