I have never spent too much time in the USA. But if they add my stays in Canada then it has, in the past, gotten above 90. This is the first I've heard that Canada = USA. I know from experience that a one-way out of the US to Canada or Mexico doesn't count as a ticket out and that's fair enough.

But anyway, the point is that, as the rules are written, someone who has spent a grand total of 1 day in the USA in their whole life could be denied access to the USA because they have "overstayed".
You get crazy results like that when you start counting stays in other countries. With any transit via the USA you could be refused entry on the way home on a return leg.