I've googled the phrase "and then some" and am told that it means "considerably more".
But just how to comprehend this? The phrase literally means "some more" -- how does it come to mean "much more"?
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And then some is used only when X exceeds Y by a significant amount, not when the margin is small:
Ben met his goal—and then some. BUT NOT
✲Ben just met his goal—and then some.
This year's figures beat last year's easily—and then some. BUT NOT
✲This year's figures squeaked by last year's—and then some.
So whenever and then some is used, the difference is already ‘considerable’. And then some adds even more (which is what the phrase means), so the sum of the two margins is, as you say, “much more”.