The degree of information sharing between US and Canada is probably deeper than most people would expect.Surely this is going to be the determinative factor. Thank god Canada and the US don't share information.
I remember back in late 2013, one person I know did a flagpoling landing (it was still allowed back then), where he would go to the border, took a U-turn then re-entered Canada, all done without dealing with CBP at all. Not dealing with CBP notwithstanding, when he checked his I-94 record online, that date was recorded as an exit from the US!
Apparently, starting June 28 2013 (as part of Exit - Entry Initiative Phase II), any land entry of non-US, non-Canadian citizen to Canada would be shared with the CBP, and CBP would record this as an exit from the US, even if the said person never entered the US at all, nor dealing with the US border agent.
This was probably also the reason why U-turn flagpoling was no longer possible nowadays, probably to avoid this kind of confusion.