Tree Peony Let-Down

So last Spring around this time I bought a tree peony at a local nursery, whose label simply read "Yellow", and which in person was a lush, double yellow flower with pink picotee edges. Gorgeous!:

Yellow Tree Peony May 2010

Unfortunately, this is what it is this spring--a single, pink flower:

"Yellow" Tree Peony May 2011

Now, don't get me wrong; it's still lovely, but...it's a bit of a let-down after last year. I've heard of the offspring of cultivars propogated from seed reverting to the cultivar's parent species, but the same plant blooming differently year to year? Huh.