I am guessing that one of the biggest differences between total compensation packages between hiring Canadians vs. Americans is that companies don't have to pay for health care costs for their employees based in Canada. This is mainly because health care is funded from our income tax.

I believe health insurance is a factor for Americans when choosing a job, but may not be as large a factor for Canadians.

From a company standpoint, this should mean that Canadians may be cheaper to hire (although, given that CDN is higher than USD at the moment, this may not hold).