I think, from a financial management perspective, we would want departments' plans to be as rigorous as possible, and we would hope not to change those on too regular a basis.
Having only two rounds, with one at a further point in the year when an expenditure total could be changed, provides clarity to departments on the remit within which they're working. There's an onus upon them to take the necessary financial management steps to live within those resources, as opposed to having multiple opportunities to revisit requests for resources from Parliament.