It is certainly true that some categories of expenses are quite difficult to forecast.
Employment insurance is a great example; that varies with the economy. What you will see is the budget will use both economic forecast and historic information to make its best projection three years out, and in some cases longer. The main estimates would pick up how much of that funding is expected to be spent in the current year. Then the Department of Finance monitors those large statutory amounts, the ones that are not voted on, on a regular basis, and where there's an update necessary, they will use the supplementary estimates process to update Parliament on revised forecasts on some of those.
If you were to look at supplementary estimates (B)—I'm going from memory here, which is a little dangerous at times—I believe there's a revised forecast in there for interest on public debt. The Department of Finance has reduced their forecast from what was in the main estimates. So we do use the supplementary estimates process both to request Parliament's approval of voted expenditures and transfers between votes, but also as a chance to update Parliament and Canadians on revised forecasts related to statutory amounts. If there's no change, you'll see nothing, but if the Department of Finance has a view that there's been enough of a change to warrant an update, we will do that through supplementary estimates.