Maybe I will start. I like to get the first word in and then give my colleague the chance to clean up.
When you look at budget projections, understand that there is a revenue and expense base of roughly $280 billion, depending on the year. Plus or minus $10 billion is a lot of money, but as a percentage it's pretty small stuff. Understand, then, that this is in context, because in the assumptions that we have to make, it's quite possible that you'll see that.
If you go back over history, the government has had a better track record in estimating expenses. Revenue is more problematic. The revenue estimates are tougher to do, but the reason varies by year. Some years it's the exchange rate; some years it's corporate taxes.
The budget is done typically two months or so before the year starts. In an environment in which the economy is changing rapidly, it is tough to estimate.
I'll let Mr. Leswick see whether he has anything to add.