suppose there are 7 people whose height > 180 but there age is <21(select based on height)
and there are 10 people whose age>21 but there height is <180(select based on age)
so we will get 10+7 =17 rows
If 7 people have height >180 and age>21
and 3 people have height<180 and age>21
then we will get 7+3 = 10 rows
so range of rows that we will get would be between 10 to 17.