I had the same thing happen to a "Portland from Glendora" bloom a few years ago. I would imagine in these cases it's due to a mutation while the flower is forming that disables one of the genes needed for pigment formation in that part of the flower, since the color of the novel half is lighter. But, always fun to see!

Here was mine:

What fun! I keep thinking of Smith’s Parish Tea rose that makes all white, all dark pink-red and dark pink-red and white blooms. Carol