The reason it says "you may be" is that it's referring to a treaty country, in your case the USA. The residency tie-breaker rules, which apply where both the Canadian and US tax authorities consider you to be a resident of their country, are in the tax treaty itself. You can find it here:
http://www.fin.gc.ca/treaties-conventions/usa_-eng.asp
For the US treaty, tie breaker rules are in Article IV (2).
Quote:
2. Where by reason of the provisions of paragraph 1 an individual is a resident of both Contracting States, then his status shall be determined as follows:
(a) he shall be deemed to be a resident of the Contracting State in which he has a permanent home available to him; if he has a permanent home available to him in both States or in neither State, he shall be deemed to be a resident of the Contracting State with which his personal and economic relations are closer (centre of vital interests);
(b) if the Contracting State in which he has his centre of vital interests cannot be determined, he shall be deemed to be a resident of the Contracting State in which he has an habitual abode;
(c) if he has an habitual abode in both States or in neither State, he shall be deemed to be a resident of the Contracting State of which he is a citizen; and
(d) if he is a citizen of both States or of neither of them, the competent authorities of the Contracting States shall settle the question by mutual agreement.
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In other words, if you have a house or apartment in the USA, and not in Canada, you're deemed to be a resident of the USA.