Because portables are quite inefficient designs, for several reasons. They are now rated differently, look for the SACC or DOE rating (I think DOE is equivalent). Eg: Many portables that are advertised as 12000btu, are only rated 7, 8, or 9000btu SACC.
Portables, almost all of'em, dual hose or single, use room air (the air you paid to condition) to cool the hot side and then exhaust that air outside.