Sorry to waste your time with a guess instead of an answer, but I noted that Air Canada's LAX-YVR fares were similarly different depending on whether you were a booking as a Canadian or an American.

The difference seemed to be that although "taxes" were included in both fares, the taxes were different. Apparently if you sell a ticket in Canada, even for a LAX origination, you can apply a bunch of Canadian taxes and fees in lieu of similar (and higher) American ones.