It is reconciled by the political reality that a government, if it is to remain in government, will have a majority in the legislature to deny any proposals for increases and expenditures it doesn't agree with. That means what the majority of the government manages to put together in the Parliament keeps a coherence to the overall budget, because it is being constructed by a particular element in the Parliament that is theoretically working together and in one direction.
Budgets are not up for grabs by individual members of Parliament. I don't think it would be in the interest of the country if they were. If one had a bidding war between the particular parties represented in the Parliament about how much should be added to the budget, that would be a recipe for fiscal irresponsibility.
A government presents a budget on a united basis, and its political strength in the Parliament ensures that the budget largely stays together. I think the Parliament can insist that the budget be presented in a clear way and that Parliament has a good opportunity, in terms of time and support, to consider thoroughly that budget before giving its formal approval.