Go value is nil in called but not caller

package main

import "fmt"

type Xer interface{}

type S struct{}

func foo() Xer {
	var s *S
	fmt.Println(s == nil)
	return s
}

func main() {
	r := foo()
	fmt.Println(r == nil)
}

outputs

true
false

Surprising (to me)!

s compares equal to nil, then is returned.
r is assigned to the return value.
r doesn’t compare equal to nil.

What’s going on?

var s *S is declared as a pointer to a value of type S. It’s not assigned a value, so Go assigns it a pointer’s zero value: nil. That’s why the first comparison prints true.

foo()'s return type is Xer, an interface. So r is an interface value. An interface value can be thought of as a (value, concrete type) tuple. In this case: (nil, *main.S). r's concrete type is non-nil. That’s why the second comparison prints false.

OK, but I’m still puzzled. Somehow the return value, or how Go thinks of it, changes in flight from foo() to main(). I wonder if this illustrates something important about… interface values? types? compilers? I’d like to understand it better.