I'm looking at the EWR-CLT flights tomorrow; why is coach shown as "Sold Out" and thus can't be bought on some flights (such as the 1:15), but the seat map shows plenty of seats in coach on those flights?

I'd like to go standby (and am Preferred), but don't want to trek all the way to EWR and have to wait for hours if I don't make it on.

Thanks.