Tree peonies are usually grafted onto herbaceous peony roots. If the root stock is flowering, you probably didn't plant the tree peony deep enough when you orignally planted it...not a big deal, just results in the rootstock flowering. As for the variety, it's probably P. lactiflora, likely an un-named variety or even the species. A tree peony propagator might shed some light on what rootstocks they most commonly use?

Simon

Thanks Simon. Another "problem" with this tree peony is that it is supposed to be Kinkaku. The yellow blooms look nothing like it. Anyone have an idea what I really have?

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Edward,
You're right. That does not look like Kinkaku(Souvenir de Maxime Cornu). But it still could be. If this is the first bloom the plant had it could be not typical of how it should and will look. If it is Kinkaku the bloom bud would generally develop just outside of the foliage and generally to the side of the plant. But as the flower opened it should have turned downward and ended up in the center of the plant. It can't be seen without manually moving the blossome to lift it. Kinkaku is probably the worse of the lot to do this. If it is not Kinkaku then I could be Alice Harding or Chromatella. These three tree peonies are the most prevalent of the cheaper yellow tree peonies.

PeonyMan, here is a picture of the second, and last bloom for this year.

Here is a link that might be useful: