idioms - the idiomatic use of "no more than" and "no less than"

I have questions about the use of no more than and no less than .
The following is the item 14. phrase of the word more from COBUILD online dictionary.

You use no more than or not more than when you want to emphasize how small a number or amount is. (emphasis) ⇒ "He was a kid really, not more than eighteen or nineteen."

The following is the item 9. phrase of the word less from COBUILD online dictionary.

You use no less than before an amount to indicate that the amount is larger than you expected. (emphasis) ⇒ "No less than 35 per cent of the country is protected in the form of parks and nature sanctuaries." ⇒ "He is lined up for no less than four U.S. television interviews."

My questions are

1. Why doesn't "not less than" have the same idiomatic meaning (larger than you expected) as "no less than" while "not more than" has the same idiomatic meaning as "no more than"?




2. How did "no more than" and "no less than" get their idiomatic meanings?




3. When do you take the meaning of theses phrases literally, and when do you take the meaning of theses phrases idiomatically.