Sometimes shoots appear on previously plain-leaved plants with attractive variegations, and these can then be propagated by nurserymen to make a new plant. But the new variegated plant might try to revert to all-green foliage because the mutation isn't stable. If left, this all-green growth, which will be more vigorous than the variegated kind, will take over the plant.
Variegated leaves are caused by natural mutations, but these mutations aren't always a stable, permanent feature, and the plant might try to revert to the original, all-green leaves.
The only solution is to cut back any all-green growth to leave just the desired variegated foliage.