This plant phenomenon, which can also happen in animals and people, is called chimera/chimaera. A chimera can occur when a mutation causes a group of cells to grow differently from the tissue around it.
Sometimes this mutation causes the plant to display a different phenotype (white flowers, instead of pink), and other times its visibly undetectable, but still genetically different.
A more common example of chimera is the vertical stripes on a Sansevieria (snake plant) leaf, whose striped area is genetically different from the unstriped portion.
This is why, if you've ever tried to propagate one by a leaf cutting, you'll notice the resulting plant won't have the same color pattern.
(Photo credit: Waylon Arms slideshow "Plant Propagation by Division and Separation")
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