That section of Hwy 403 in question opened in 1963, and was among the first six lane highway segments to be opened to traffic in Ontario.
Back in the 1960's, even in Ontario, most six-lane (or wider) highways in Ontario lacked left-side shoulders. Think of highways such as the QEW through Mississauga, or Highway 400 between Toronto and Barrie. It wasn't really until the late 1970s or early 1980s that Ontario started building every six lane (or wider) freeway with fully paved left and right shoulders.
What separates Ontario Freeways from those in Quebec, is that Ontario has retrofitted so many of its older freeways with newer, upgraded design standards while Quebec has not.
For example, not that long ago, the 400 had no left side shoulder where it ran through both Toronto and Vaughan:

(Sorry for the image size),
Today, it obviously does:
Same for the 401 through Scarborough and Pickering for example:

(again sorry for the image size),
while today the freeway is totally unrecognizable from 20 years ago:
Even Québec has significantly updated its standards, the province just never upgrades any of its existing infrastructure.
The A-25 extension has left-side shoulders despite the fact it is only of a four-lane configuration: