In general, I would say that in the main estimates statutory programs are forecast and there are updates throughout the year. We may see one forecast in the main estimates; we may see updates to those forecasts in supplementary estimates, and we may see updates to those forecasts in budget documents. It is fundamentally a forecast, as you said, and the actuals will invariably differ from the forecast. Now, by what margin is another matter.
In terms of a specific program and the forecasting methodology that goes into those programs, I would not be able to answer that question. I think you'd have to refer it to the department that administers those programs, unless one of my colleagues can offer something.
But I think that's a question you'd best address to the department.