Sounds like the branch is producing a sport, probably related to its ancestors. It is usually limited in scope (a branch for example). You can prune the branch if it bothers you.

I had the same thing happen with Eleanor McCown (sp?). EM is supposed to produce white blooms with pink stripes. While still young (2-3 years old?), one branch started producing blooms that were all white, all pink, etc. I decided to let it be and see how it progressed but, sigh, my dogs had to remove the plant because it was in the middle of a "large interstate highway between them and the squirrels favorite hangout". Hee hee hee! They stomped on it and broke the main trunk in half. Oh well. I have waited to see if the local nursery brings this plant back for sale to see if it does the same thing or not. I would not mind if it did because it looked nice and unusual.