Wild arsed guess.....I'll bet that multiple layers "slip apart" more quickly and smoothly than a single layer would allow the string to slip off (only the string moving, no layers moving).
To try to describe it more, one layer would "stick" to your fingers while the string tried to slip off of the face, both surfaces creating friction. If you have multiple layers, those layers can more easily "slip apart" allowing the string to slip off the fingers more quickly and smoothly. Does that make sense?
Just some intellectual conjecture on a Friday night.
homebru