I don't understand why the address has been changed between "Sa:" and "Sb:" by assigning another anonymous hash. I expected "Sb:" would be the same as "Sa:", since i was using the same var. [Emphasis added]

That's like saying

In the code
    $x = 4;
    $x = 7;
I don't understand why the value of  $x is 7 after the second assignment to $x: I'm using the same variable.

As others have noted, it's vital to distinguish between a variable, the content of the variable, and, if the content of the variable is a reference, the referent of the reference. References are tricky.