Activatepackage() / script calling hierarchy question

Here's a quick question for all you scripting gurus:

I've been going through all the realm wars scripts and I'm having trouble understanding exactly how the function calling hierarhcy is affected by activatepackage().

Specifically, my problem is this:

in onStart() function in rw/main.cs, why does

Parent::onStart();

call the onStart() function from common/main.cs?

I know that common/main.cs was exec'd before rw/main.cs but
why does this make the onStart() from the Common package the
parent of the onStart() function from the rw package?

Thanks for any help.

-Pascal