[racket] Confused about define-values(-for-export) forms in define-signature

The define-signature form allows define-values and define-values-for-export forms in its body. According to the documentation the define-values form should prefix any unit that imports the signature, and define-values-for-export should suffix any unit that exports the signature, but the actual behavior doesn't seem to match the documentation.

I can define the following signature and unit:

(define-signature x^
  ((define-values-for-export (x) 5)
   x
   ))

(define x@
  (unit (import)
        (export x^)))

(define-values/invoke-unit x@ (import) (export x^))

This makes sense to me, because the definition for x is available within the x@ unit, but it must also be listed in the signature by name in order to actually export the value, so that x is defined when invoking the unit with define-values/invoke-unit.

What confuses me is that I can also define the following signature and unit:

(define-signature y^
  ((define-values (y) 5)))

(define y@
  (unit (import)
        (export y^)))

(define-values/invoke-unit y@ (import) (export y^))

This does introduce a binding for y with the value 5, but I don't understand why. Since the y^ signature is being exported instead of imported I expected that the value of y should not be available in the y@ unit, and furthermore there is no need to declare y in the signature as a variable which the unit must define. So why is it the case that this unit defines and exports the value of y, when according to the documentation this shouldn't happen.

Thanks
Dan