@Christine, variegation in most plants is caused by changes at the cellular level of the leaves. These mutations can be naturally occurring, of engineered by man. In addition there is more than one type of variegation that can be caused by more then one natural change. Variegation is patterned, or non-patterned (think a striped draecenea vs a variegated Monstera)
Variegated plants usually are weaker than non-variegated ones, and need more light. The more light they get, the better they can photosynthesize with the GREEN parts of their leaves (the non-green parts don't photosynthesize). And the better they will maintain their variegation.
Variegated plants can revert from being variegated to not being variegated for a number of reasons, the most common being as a protective mechanism that will give them more usable green leaves to ensure their survival. But with plants like Epipremnum (which is what your 'pothos' is) it is most commonly due to being grown in low light (which is what makes this plant so desirable as a houseplant in the first place).
The variegation will start to reassert itself sometimes if the plant gets more light. The genes are there, and will express themselves if given favorable conditions