I don't know the answer but it's probably the variegated part taking over. I wonder if you took a cutting of the white part, if the resulting plant would be all white.

Most variegated plants are actually a combination of two plants, one variety normal green, and one mutated to have no chlorophyll (albino). The cells grow peacefully together in the buds, and usually make up their respective parts of the leaves with no muss or fuss, but occasionally one or the other takes over the growing point, and the resulting shoot is all one color. Unfortunately, when the albino part takes over, it can no longer use sunlight to make sugars, and if separated, soon dies.