Difficult to explain!

I was deadheading some roses on the arch when I noticed this pair of white blooms on a rose which produces pink flowers. I have no idea why this should be unless a stem from a different rose was grafted on by the grower, which seems highly unlikely.  ( No faffing about with photoshop, or similar - honest! ) Another very pleasantly warm day but with the promise of some rain tomorrow which might encourage me to  transplant some bedding plants.