Why is nil != nil in Go sometimes

wtf.go

package main

import "log"

// some random interface
type User interface {
	Name() string
}

// some implementation of that interface
type inMemoryUser struct {
	name string
}

func (user *inMemoryUser) Name() string {
	return user.name
}

// something that returns our implementation
func findInMemoryUser(id string) *inMemoryUser {
	// (always returns nil for this demo)
	if id == "cool-dude" {
		return &inMemoryUser{name: "Cool Dude"}
	}
	return nil
}

// something that returns our implementation as the interface
func FindUserByID(id string) User {
	return findInMemoryUser(id)
}

func main() {
	user := FindUserByID("random-man")
	if user != nil {
		log.Printf("not nil %+v, let me say hello:", user)
		log.Printf("hello %v", user.Name())
	}
}