It's in its essence, very simple : you don't get paid as much as you should when you contribute on "lucky" blocks. You have to contribute to the "unlucky" blocks too to get all your rewards. Thus eliminating the motive for pool hoppers (that try only to contribute to "lucky" blocks).

It's also, in my opinion, better for the mood to see the reward going up more than expected on otherwise long blocks.

Now, of course, the math behind is not simple, but the principle is really clever and easily understandable.