Some people think they are poly because they've had group sex or sex with several different people within the same time frame. Having group sex or a threesome, or sex with Dan on Tuesday and John on Wednesday does not make you polyamorous. It only means you've had sex with multiple people.
Being polyamorous means we've emotionally committed ourselves to loving relationships with more than one person. Obviously, investing ourselves emotionally to one person is already difficult enough, but doing so with more than one person is even more taxing.