No question, McGrath---much of it is a 'perfect memory' rather than an absolute and 'magical' knowledge of the pitch.

And I appreciate your response, Highlandman. I actually wouldn't have trouble singing 'Happy Birthday' in any key because I'm musical and have some relevant pitch.   

But if there is a more difficult song I've learned the chords for in one key, it can sometimes (but not always) be a challenge doing it in another key.   Whether that's on keyboard, guitar....and occasionally vocal.

So I don't think that perfect pitch totally cancels out relative pitch. I think it's more of a case of not needing it when you have perfect pitch.......and if you don't use it you lose it (so they say).