b) I'm no statistician
I think they're looking at it historically. If they know that every year the total number of bookings for the season was roughly 2x the number of bookings received by end of January, they use that as a guideline for the coming season.
But I agree, the competition is -ahem- hotting up (or it will be, come May) and it's difficult to imagine all the Thomas Cook rooms being taken by other companies.