Singles. 21st Century Breakdown was a single. St. Jimmy and Are We the Waiting were not. Brain Stew and Jaded were separate tracks on the album, but flow into each other, were put together as the single, and are always played together live.
Witticus: "GeeNef speaks to me like schizophrenia, his words touch me where my priest could reach."
If I remember the podcast correctly, the other songs were combined because that's how they got the masters, while Song of the Century and 21st Century Breakdown were received separately. They could have combined the songs, but chose not to so that the rest of the band wouldn't have to stand around doing nothing for the first minute (even though bass and drums have to wait longer on F.O.D.).