I suspect the answer is less ambiguous (less likely to receive 3 perfect 100's) if the respondent has taken doctrinal classes for their belief. For example, I had a single 100 for Conservative Protestant. Yet one of my friends got 100 for Conservative Protestant, Islam, and Orthodox Judaism.
What happened?
While me and my friend have quite similar beliefs, I have taken 7 theology classes at the local Bible college over the last 3 and a half years (one 3-hour evening class every semester). The professor holds a Masters of Divinity from Yale, and an undergraduate in philosophy from Azusa Pacific (I think).
So my beliefs were refined, whereas my friend's beliefs were more general. All 3 religions in which he scored 100 are of the theistic worldview. He simply didn't know to articulate his Christianity given the survey questions. I'm saying this from a position of knowing what my friend believes (from conversations), so I'm not imputing to him a specific belief system when he really does have a general theism bent.