I worked in regulatory compliance at a big food company for several years and some of the silly label language is simply young marketing people trying to make a name for themselves in their company, some are desperate attempts to stand out in a crowded marketplace and some are caused by truly idiotic regulations - Prop 65 is a good example.
(California simply created an extortion industry with a silly product labeling law called Proposition 65. Hundreds of activist groups, lawyers, consultants and state regulators have very well paying jobs because of this one law that sucks money of of the rest of the country via the higher costs of compliance we all pay on the products we buy that have to comply with Prop 65 - and the cost of paying off the Prop 65 industry.)
Gluten contains two main proteins Gliadin and Glutenin. It is the Gliadin which causes most problems. Wheat is the commonest source, but it is also found in Rye, Barley, and Spelt, in fact any grass of the tricum family.