It's because the RC is not "sent", when you delegate you give max RC and current RC.

Current rc is the rc you use (the actual unit). Max rc is what is used to define up to how much rc you can have and at what rate you regenerate those.

So if I delegate 100 rc to you, you can use those 100 rc (current rc) and then they will also regenerate on your account for as long as you have the delegation on you.

So the delegator won't in fact start regenerating the delegated RC until he undelegates. I don't know if I'm making sense, if not tell me I'll add more examples :)