Flights are oversold to compensate for no-shows, based on historical flight data for a given market, season, day of week, etc. An aircraft may hold 100 passengers but historically it may have 5% of no-shows at departure, so the airline will oversell the flight to compensate for those no-shows. At the time of departure, there may be more or less no-shows than planned, or there may be misconnects (i.e., connecting passengers whose inbound flights arrived later than scheduled). No-shows and misconnects = good news for non-revs. So, yes, it is very possible that non-revs will be accommodated on an oversold flight.