J, well, I didn't say that those people are necessarily healthy - just that they don't have Crohn's.
A positive ASCA IgG by itself can be found in celiac disease, behcet's disease, other forms of colitis besides crohn's, and sometimes, yes, in healthy people too.
Also, the definition of "positive" is somewhat arbitrary. What levels exactly? Studies I've seen define a range from 0 to xx as being negative, from xx to yy as being marginal, and from yy to above being positive.