The variegation required to get an all white (or all yellow or all green or all pink) leaf on any variegated plant is just a natural variation of non-patterned chimeral variegation. Its not rare. The mutation in the leaves of whatever plant it is provides genetic ability to have any amount of any color the plant naturally has in any leaf or in the stem at any time.
This Philodendron 'Brasil' (a common houseplant in a hanging basket) has all green leaves (the equivalent of all white) some all more yellow leaves (the third color in this variegation) and normally tricolored leaves.
This philodendron "Rising Sun" has all green leaves (not the norm) in addition to the yellow and green variegation that it is known for
This Lime Zinger Xanthosoma is supposed to have all-chartreuse leaves but occasionally it will throw and all regular green leaf, or a color-break leaf like this one
Even flowers can do this...the flowers on this single stalk of Princess Victoria plumeria are all different, some variegated, some solid, and even the solid ones are different colors.
Just because it has never happened to you doesn't mean it does;t happen on a regular basis to other people. This type pf atypical variegation is just a product of the fact that non-patterned variegation in plants can take any form... solid, half and half, speckled, splattered...