I finally decided to get to the bottom of this. I sent an email to Montie Brewer, and one his assistants responded by email, saying that someone from the AC IT department will contact me. I spoke to that person, who admitted, that legal connecting flights (domestic, transborder, international) can be booked through the "round trip" or "one way" options, and one will get a combined fare. However, if one tries to obtain a fare for a legal connecting flight (either the same combination of flights that do pop up on the screen with a round trip/one way search, or other combinations that are not displayed through those 2 search processes), one will only get an added fare. (We did some test fare searches (simultaneously) of connecting flight itineraries, using the multisegment booking, and she got the same results as I did - in terms having the final fare equal the sum of the fares of the individual segments).

She indicated that the aircanada.com web site has that limitation, for now. I then sent an email back to Mr. Brewer's assistant, and she agreed that this limitation does exist, but she hopes that in the future, whenever the web site is upgraded, this functionality will be improved.