Help me understand my Array.IndefOf problem.

So base on my understanding of Array.IndexOf(value) is that it will get every index that matches the "value"

But how come in the pic below that its only updating the FIRST instance of the Array.IndexOf(var) while it loops through the array.

What i expect base on the pic above is that if var=13 , then all Array.At(Array.IndexOf(13),1) should + 1.

So both Array.At(1,1) & Array.At(2,1) should be 2 and not just Array.At(1,1).

Let me know if I'm understanding this incorrectly..