Clematis montana - or is it?

At this time of year our fence is smothered by pink and white clematis montana flowers.

The other day I noticed that among the normal four-petalled flowers there were several with five or even six; one can be seen towards the bottom in this photo This is only happening on the white montana. Counting petals on clematis flowers? I need to get out more.

With this photo I have used the HDR technique, taking three shots with different exposure values and then blending them in Photoshop. When I compared this with the original photo that had no exposure compensation the details are much clearer. I'm a convert.