Cathay Pacific Asia Miles - Difference in Fares: Canadian Gateway vs. US Gateway

Can someone explain why CX fares in Y to HKG are so much more expensive out of YVR and YYZ as compared to out of LAX, SFO, or JFK.

Take this for an example of fares North America-HKG departing 3/13 and returning 3/26 in Y class:
YYZ-HKG RT: $2000 USD
versus
JFK-HKG RT: $790 USD
Fare Difference: $1210 USD

YVR-HKG RT: $1700 USD
versus
LAX/SFO-HKG RT: $740 USD
Fare Difference: $960 USD

Why go so far south to find a better alternative to YVR or YYZ? You can save several hundred bucks on average by driving two hours to SEA or BUF.

And interestingly enough, first and business class fares are just the opposite, i.e. ex Canada is much cheaper.

From what source are you comparing the prices with?

T/As in Canada offer similar fares out of Canada that are comparitive to the US ones.