I suppose the fact that eating out in Canada is expensive has an effect as well.
Example - we like to eat at the Keg Steakhouse. In Florida especially there are several equivelent types of steakhouses. (Can't remember the names). For a steak dinner with a couple of drinks costs well over $100 here. We find the equivelent meal in the US at least 30% if not more, less than here. Since we usually leave about the same amount of grautity in either place it I suppose we are paying a higher % in the states than in Canada.
Although minimum wage for servers is less than regular minimum wage here, it's certainly way higher than it is in the states. I know that the average server here is making pretty darn good money and certainly isn't declaring it all on income tax.
That all being said, we still give a gratuity based on a combination of two things - service and cost. Good service - decent gratuity. It ranges all the way down to lousy service - no gratuity and also a complaint to management (which I've done a few times.)