I think UA faces higher incremental for a number of reasons:

1) Stage length. (But this is counteracted, to a degree by the higher landing costs of the A/C capable of longer stage length).

2) Congestion. On average, UA aircraft face longer approach holds than AC aircraft. While this is a marginal increment, it is, nonetheless, a direct one.

3) Scale. US airports are more crowded, and can, therefore, distibute costs accross a higher base.

4) Financing: Many US airports are financed through bond issues or other forms of private/public investor financing. They do not, therefore, have the same level of debt servicing charges as Canadian airports would.

Pull out the financial peformance figures for CX and NH. They operate out of the most expensive airports in the world, but also in jurisdictions where they have to import all their fuel. The numbers should be interesting--not only for their relative ratios, but also as indexed against RPK's.