Downcasting third layer class

I am preparing for the SCJP exam and I have run across this snipet of
code that is confusing me.

1. class Alpha { void m1() {} }
2. class Beta extends Alpha { void m2() { } }
3. class Gamma extends Beta { }
4.
5. class GreekTest {
6. public static void main(String [] args) {
7. Alpha [] a = {new Alpha(), new Beta(), new Gamma() };
8. for(Alpha a2 : a) {
9. a2.m1();
10. if (a2 instanceof Beta || a2 instanceof Gamma)
11. a2.m2();
12. ((Beta)a2).m2();
13. ((Alpha)a2).m2();
14. ((Gamma)a2).m2();
15. }
16. }
17. }

The way I see this there are three levels of inheritance

Alpha
  +------Beta
             +------Gamma

I can understand why lines 11 and 13 fail to compile. Both are
instances of Alpha trying to call an m2() method, which type Alpha
does not have.

I can also understand why line 12 is fine. It is being explicitly
cast to type Beta, which does have an m2() and therefore works fine.

My confusion is with line 14. This line compiles but throws a
ClassCastException at runtime.

Since Gamma extends Beta and Beta contains an m2() method, then Gamma
should also contain an m2() method. Right? The method is not private
in Beta so it should be inherited.

Why can I cast to Beta with no problem, but not to Gamma?

I've looked through texts and tried to find an explanation. All of
the examples I've found only go 2 levels and that makes perfect
sense. I cannot find any explanations with 3 levels that might clear
this up for me.

I feel I am just overlooking something obvious, but I've stared at it
so long I am lost. Some guidance would be appreciated.

Thanks.