Yes, some people do have high IgG levels to certain foods but do not have any symptoms at all. This is most likely due to their immune system being extremely efficient at clearing away the antigen-antibody complexes before they have chance to be deposited in the tissues and cause a problem.
It is true that certain common foods are more likely to show a positive result e.g., wheat, dairy, soya. The reason for this is that these foods are consumed on a regular basis in our diet and along with digestive issues this can increase the likelihood that the body may react to them. While an elimination diet will always be the gold standard, the benefit of doing a food sensitivity test is that it will be able to quickly show which foods eaten on a regular basis are a potential problem. In addition, the tests can highlight more unusual foods which a person may not have considered removing from their diet.