why "and then some" means considerably more?

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"?

• What it means depends on context. There's no inherent understatement or overstatement in the expression. It merely means "more than expected" or "more than required" or "more than the number stated". But if the "more" is only a little bit more, that fact will usually be stated as "a little bit more", so perhaps you can assume it means "noticeably more".
– user21497
Apr 16 '13 at 14:50
• Broadly I agree, but in the context in which I've usually seen this phrase used, it tends to mean noticeably more, so I generally consider it understatement.
– njd
Apr 17 '13 at 9:30

I think it's a case of using understatement.

Taken literally, "and then some" could be taken to mean "plus a little more".

But the way it's used, it generally means "plus much much more."

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”.