In some cases, the way the cereal or grain is made up may cause a sensitivity versus the gluten itself.
While we don't test for gluten on its own and only as part of the grain, gluten sometimes only makes up a certain percentage of that particular cereal or grain compound. Ex: in some cereals or grains, there may be a higher percentage of gluten, which may be why you are sensitive to that particular cereal or grain.
In other cases, other compounds found within cereals and grains can affect the body's cells. Ex: There is a well-known protein called "Gliadin," commonly found in cereals or grains; while we may not test for "Gliadin" at this time, this protein has been known to contribute to inflammation.