The reason it gets refilled is that Linux (of which Android is a derivative) behaved MUCH differently than say, Windows. The philosophy is that empty RAM is wasted RAM, so the kernel will leave apps open, but not running (whereas with Windows, open = running). This is how you can hit the home button, send an SMS, and jump back to your previous app and find that it's still open. It can be quite advantageous.
Please hit the "thanks" or "like" button if I've helped.