I think the answer might be a bit more simple than you’d like but i quickly looked through the CAR’s and I don’t think Canadian operators are required to display the registration under wings like in other countries
.
From section
222.01:
(h) the display of marks on the bottom surface of the wings is optional,
In the
UK it looks like displaying it under the wing is not optional and that it must be visible to all viewers even on the ground:
Paragraph 1(2) of the Schedule says that the nationality and registration marks shall be displayed to the best advantage taking into consideration the constructional features of the aircraft; and shall always be kept clean and visible. The ‘best’ advantage is the best advantage of an external viewer either on the ground or in another aircraft.
My bet is AC doesn’t display the reg under the wing because they don’t have to while other carriers from other nations are required to display it based on their own countries regs
.
Edit: Just reread your post and saw you knew it was optional. No idea beyond that why they don’t display it
.