It turns out that Expedia for TD is only selling tickets to some Air North flights, but not to all. I don't seen any reason or rhyme to this.

For example, there are three Air North flights from Vancouver to Whitehorse on March 22. Expedia for TD (and Expedia.ca) lists only two out of those three. I suspect that is the usual trimming of flight options that some algorithm at Expedia for TD does.