Sometimes plants will throw out a sport on one branch. I haven't seen it happen on any of mine, but some that are in commerce started that way, so I know it is possible with clematis. Sometimes a seed will sprout, and because of the multiple stems and twining vines it can be difficult to tell if it is the original vine or a new seedling.

Thank you for your reply. I was wondering if it was a sport as the anthers are similar but flower is certainly different. Be curious if I get more white flowers.