Why invariant generic type parameterized with subclass of upper bounds fails to conform?

Given:

class Invar[T]
trait ExtendsAnyref extends AnyRef
def f(a: Invar[ExtendsAnyref]) = {}

The following is erroneous

scala> val x: Function1[Invar[_ <: AnyRef], Unit] = f
<console>:13: error: type mismatch;
 found   : Invar[ExtendsAnyref] => Unit
 required: Invar[_ <: AnyRef] => Unit
       val x: Function1[Invar[_ <: AnyRef], Unit] = f
                                                    ^

Why?

I understand that in Scala, generic types have by default nonvariant subtyping. Thus, in the context of this example, instances of Invar with different type parameters would never be in a subtype relationship with each other. So an Invar[ExtendsAnyref] would not be usable as a Invar[AnyRef].

I am confused about the meaning of _ <: AnyRef which I understood to mean "some type below AnyRef in the type hierarchy." ExtendsAnyref is some type below AnyRef in the type hierarchy, so I would expect Invar[ExtendsAnyref] to conform to Invar[_ <: AnyRef].

I understand that function objects are contravariant in their input-parameter types, but since I use Invar[_ <: AnyRef] rather than Invar[AnyRef] I understood, apparently incorrectly, the use of the upper bounds would have the meaning "Invar parameterized with Anyref or any extension thereof."

What am I missing?