Canada is listed with a three hour recommendation for check in due to the long times possible going through formalities.

Agree that the web site does not list a different absolute cutoff, thus the OP was there well before the published cutoff. However, the reality is that the agents are going 'between countries' as US immigration is ahead of the gates and if those agents have to work the gate, they would be 'on the American side' of the border. Not sure what they have to go through to move back and forth but that could explain why they shift earlier in spite of the published cutoff.