This has to do with how the US Transportation Tax is calculated. The US government wants to prevent travelers in the US from originating from a Canadian Airport to "avoid" US taxes. These rules were put in place back in the days when paper tickets existed and travelers typically bought their tickets from a local bricks and mortar travel agency or airline ticket office in their home country. Now with electronic tickets and online booking sites, one can easily get around these rules by simply booking from the website that gives you the best fare.

For domestic travel within Canada which involve an airport in Canada less than 225 miles from the U.S./Canada border, tickets sold from a U.S. point of sale (this includes Air Canada's U.S. website) are assessed the 7.5% the US Transportation Tax on the base fare in addition to the usual GST/HST depending on province of origin.

As for travel from Canada to the U.S. or vice versa which involves an airport in Canada less than 225 miles from the U.S./Canada border, tickets sold from a U.S. point of sale (this includes Air Canada's U.S. website), the 7.5% US Transportation tax is also assessed. If the same ticket is purchased from a Canadian point of sale (eg. Air Canada's Canadian website), the US Transportation tax is reduced to a flat USD 21.10 each way.