Hm... Just looking at the categories, I think I can see it. Does it say anything in the INFJ's about spiritual beliefs? It seems to me that Sensing, Thinking, and Perceiving categories would be more keen to go with the 'show me proof' crowd. I'm not so sure that Introversion v. Extroversion would influence that as much. Oh, and 'most likely' doesn't always mean much.
Last time I took the test, I was an INFJ. I'm also atheist, though I'm sure a sample size of one doesn't actually help anything. I do have a strong sense of spirituality, but I come to it through means other than stand-here-say-this religion.
If I had to guess based on my own experience, I'd say that INFJs perhaps internalize their spirituality, perceive their sense of spirit come from within, rather than ENFJs externalizing it, perceiving their sense of spirit coming from without? Like, INFJs might have a higher chance of believing that they're dealing with themselves, and ENFJs believe they're dealing with forces outside themselves? It's knee-jerk, probably wrong, but that's my thoughts on it right now.