Variegation is a many faceted thing. There are different types of variegation, but this is chimeric variegation meaning there was a spontaneous mutation that occurred. Chimeric variegation has many forms and can be stable or unstable. Since the mutation has to occur in meristem tissue (the root tissue and the true stem tissue) and be translated out through the rest of the plant (petioles, leaf blades) the extent of the mutation determines how it translates.
Highly variegated plants like the ones above will have a high degree of mutation that translates through many cell layers. The more times the mutation 'goes all the way through' more complete cell layers, the more spots, streaks, and patches a plant will exhibit.
The heaves that are already patterned will not turn back green. But you may never get another variegated leaf, or you may get one every once in a while, or you may get them in successive spurts.
