Why can't overriding methods throw exceptions broader than the overridden method?

Question

I was going through SCJP 6 book by Kathe sierra and came across this explanations of throwing exceptions in overridden method. I quite didn\'t get it. Can any one explain it

Answer 1

Rule of handling check and unchecked exceptions on overridden methods

-When parent-class method declares no exception, then child-class overriding- method can declare,

 1. No exception or
 2. Any number of unchecked exception
 3. but strictly no checked exception

-When parent-class method declares unchecked exception, then child-class overriding-method can declare,

 1. No exception or
 2. Any number of unchecked exception 
 3. but strictly no checked exception

-When parent-class method declares checked exception, then child-class overriding-method can declare,

 1. No exception or
 2. Same checked exception or
 3. Sub-type of checked exception or
 4. any number of unchecked exception

All above conclusion hold true, even if combination of both checked & unchecked exception is declared in parent-class’ method

Ref