All zeros doesn't always mean oversold. They often zero out flights when earlier flights are oversold, or cancel, etc., so they can accomodate those displaced passengers. Also, even if they don't 'zero out' the flight manually, and they actually sell all the seats, not all flights are authorized for oversell. Furthermore, if the front cabin isn't zeroed, but the back is, they often accomodate the overflow with op-ups, thus avoiding bumps.

In general, they are very reluctant to bump people before an hour before unless they're way oversold. SMS will usually give you a heads up if you ask if they're that deep in it.