What does "and then some" mean and why is it used that way?

Solution 1:

The Macmillan dictionary has this entry for and then some:

: used for emphasizing that there is more in addition to what you have mentioned

The house must have cost half a million dollars and then some.

Macmillan also notes that and then some is generally a spoken expression.

Solution 2:

Is this a quite accurate definition? (with the stress of "considerably more"). Why is it like that, I wonder -- is it a short form of something, such as "and then some more"

In a word - yes.

Solution 3:

"And then some" is an idiomatic expression, intentionally understated for effect.