Th sign-up sheet was that bit of paper in June where you had to tick whether you thought we should leave the EU or stay in the EU.
It is a forecast upon which this budget is predicated: as such it is having a significant impact on spending on everything else. If you have a forecast for heavy rain and cancel sports' day, sports day remains cancelled even if the rain turns out not quite so bad as was forecast.
So yeah, it might turn out to be £56 or £65 billion, and you're right, this is a prediction - but in practice the distinction is meaningless, because a) it's already in the ballpark of tens of billions and b) this is the figure that everything is based on.
I am sure many, many Leave voters knew all along that this would be the case, and of course not all of them believed there would be spare money. I'm utterly convinced of it. It does beg the question of why on earth they thought it was a good idea, in that case, though. At least the muppets who thought we'd have extra money had a valid, if utterly unsubstantiated, reason for their decision.
Doesn't really matter if you're a homogenous mass or not - the collective result is the same, thanks to all Leave voters whether motivated by racism, disgruntlement, some kind of genuine feeling about something, or love of 'sovereignty'.