Unless genetically disease can be 100% ruled out (sibs or parents have known mutations), I think the more likely possibility is an undiscovered mutation, like Heather states.
However, if a carrier can sweat a 54 (as my son did) and go on to get a perfectly normal NPD test, which would indicate functional CFTR... I would have to think that an additional 6 points on a sweat test to go from high borderline to positive is not an impossibility, realistically speaking.
eta> there are other health conditions that can cause an elevated sweat test. However, they come with symptoms of their own and this doesn't seem applicable in this situation.