Hydrangea, blue and pink

We have several hydrangea plants in out garden. One is a white climbing version. But I’ve always liked the blue and pink ones the most.

Now I know that the colour of the flower heads is meant to be partially determined by the pH of the soil (blue in acidic, pink in alkaline).

So how on the same plant have I got blue and pink flower heads so close together. 

The best of both worlds perhaps.

And quite intriguing. Explanations welcome.