Probably a silly question

[eluser]theprodigy[/eluser]
I'm just a little curious here, but:
When you extend the majority of CodeIgniter's core libraries, it just works. You still reference it the same way.

For instance, when you create a MY_Router to extend the functionality of CI_Router, you still only reference it with

Code:

$this->router

You don't have to use

Code:

$this->MY_router

Why is it then, that when building controllers and models, you have to use MY_ in the extension (If extending MY_Controller or MY_Model). Is it because of the syntax of "extends" and it not being a class instantiation?