Its probably for the same reason that yellow Welsh poppies sometimes sire orange seedlings and black cats can have tabby kittens.(And same with aquilegias as you say) It depends on the genetic makeup of the plant and also that of the pollinator. I like the white ones don't you?

It's probably a seedling. While most campanulas are blue, most have a white-flowered version as well (and even rarely a pink form)

Albino flowers--white instead of blue, pink or yellow--is the most common mutation affecting flower color. The biochemistry of flower color is so complex, any of a number of mutations could disrupt the chain of reactions leading to flower pigment. And, of course, a flower without pigment is a white flower.

This one is specifically Campanula percicifolia and it does frequently have white flowered forms.

Thanks every one, much obliged for the information. There's nothing simple about gardening is there, but that's what makes it so interesting.