I too asked the same question in the beginning of my flushing history.
Then I flushed four times with two weeks intervals having different results each time. Different size, amount, and colour of stones, which makes your first suggestion false.
My fourth flush brough very few stones of a really small size. I believe I finally got rid of all of my old stones. So since then I flush every two or tree month when I feel I need it.
What I understood from my own experience is that gall stones are constantly forming in our liver and gall blader. When liver gets clogged with stones it stops to eliminate cholisterol from our bodies, stones make obsticle for normal bile flow, and all this causes a lot of digestion problems and other health problems.