Because of course mistakes are never made, even to the point that tours end up getting cancelled.
Not that I think that'll happen here, but let's not pretend that the people booking these tours know all. They're guessing like the rest of us.
It's not Sam sitting in the pub pissed booking places for a laugh.
I'm pretty sure these booking agents/ management people know a little about what size venues to book.
All will sell out or be almost sold out.