questioner wrote:

I understand that set A and set B are composed of 5, 6, 7, 8, 9, 10, 11 and 7, 8, 9, 10, 11 ... so on, respectively. ... What do I miss?

You've missed the word "inclusive" in the question statement, which means that the border values are also the elements of the sets.

Therefore the sets are:

{4, 5, 6, 7, 8, 9, 10, 11, 12}

{6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20}

The intersection of the sets is

{6, 7, 8, 9, 10, 11, 12}

It contains 7 elements.