I think in this case, the department did, in order to support the budget, put forward studies or look at the impact of the budget as a whole, and it grouped the measures together.
I can point you to the budget plan for 2016. Annex 2 of the budget sets out the economic impact of budget measures. There it divides up measures into broad categories, so that the corporate income tax measures were grouped together. At that scale, they were found not to have a kind of particularly material impact at that high level.
Very much, the government does consider the impact of measures, but budget-making is about making choices between alternatives. When a government puts together a budget, it looks at the economic impact of the complete ensemble of measures. That's what's set out in the budget, with just those sort of macro-level breakdowns.