Nah, it dates back to the early years of the theatres and the vertically integrated monopoly Hollywood had over Canada. Pretty much all Canadian film distribution was American owned so they just included it in the domestic totals.
And because of the sale percentage difference in domestic vs international, it just makes sense to keep including it (i.e. Domestic movies might see the Studio take in 50% of box office sales and the theatres see 50%, Internationally it differs greatly with the studios seeing significantly less in China, for example).
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