Hmmm, I notice the ANS says this about the VALUES type specifier:
I consider this to be a flaw in the ANS. The CHECK-TYPE macro takes a _place_ as its first subform, and _place_ is defined in the glossary in terms of _generalized reference_ which in 5.1.1 contains text mentioning values plural in a couple places, e.g.
the ultimate result of evaluating setf is the value or values being stored.
I think during the proofreading of the many ANS drafts no one except me ever checked this issue for consistency, and I did it 21 years too late.
If god had intended standards to be such precious things, he wouldn't have made so many of them.