What is the LIBRA-SCORPIO CUSP?

If you are born on the cusp and have, for example, Mercury venus and Pluto in Scorpio, is that the reason you are considered to be on a cusp, or does the sun have influence from the next sign? I am asking this, because was i born earlier that month( not on the cusp) i would still have those exact same planets in Scorpio. Since they say that being on the cusp means having more of the next sign in your chart, how come being born earlier would have the same planets in this sign?