All of the cats I TNR'd came back within a day or two.
They spread the word

Living out there is tough, and any comfort they can find in terms of shelter, food & even medical care will be accepted eagerly.
As to where they all come from I can only give you my personal opinion...irresponsible people. People who just leave their cat behind when they move, people who 'dump' their cat when they grow tired of it, people who think they don't need to do anything as a cat takes care of itself, people who don't alter their cats and let the litters run loose...