That's a good question, and one we look at regularly.
First, non-resident income tax includes, among other things, the tax that the Canada Revenue Agency collects from entities that are not resident in Canada, but have revenue in Canada. These are not individuals. They're usually entities such as companies or trusts. So that tends to be pretty stable because, if taxes go up, their activities will move.
As far as corporate income tax is concerned, there have been a number of positive surprises over the past few years at the federal level, but also in most provinces where corporate income tax has been much higher than the models had been suggesting for a number of years. So when it comes to corporate income tax forecasts, we don't expect positive surprises all the time. That's why the forecasts are rather stable on that side.
Next, personal income tax generally tracks GDP growth quite well. This is fairly predictable, given that the base is fairly broad. These are millions of individuals, unlike corporations, where a much smaller proportion tend to be subject to tax.
This goes a long way to explaining why corporate income tax is expected to be relatively stable. It's because it has been much higher than expected in recent years, while personal income tax is expected to grow with nominal GDP growth.