I think i've read that it isn't always possible for some manufacturers to always know for certain on a regular basis whether some of their flavorings or other ingredients are gluten-derived, and if so, how much of that would be considered still gluten free (parts per million, etc.)--or if it's just an ever-changing incoming supply of various chemicals and stuff they use...it would be impossible for them to always know.
Right now, in the U.S., normally, corn or potato are used as starches in the vast majority of processed foods. Corn is also used to make citric acid here. This is because corn-manufactured products/ by-products are extremely cheap right now in the U.S. (the reason why high fructose corn syrup has begun to appear in just about everything)...but, I worry this might change soon, becuase of corn now starting to be the major crop used for fuel...namely, ethanol. This is just my own fear, here...but I worry that the day might come when corn gets to be expensive, and they start using wheat to manufacture all of these starches, fillers, flavorings, preservatives, caramel coloring, etc., and the labeling might take a while to catch up with that. That could hurt a lot of people...IF (again...my own fears!) that should happen.
Big food companies mostly use whatever is cheapest, from one purchase to another.