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.).
Plus there's one 21st song that has a minute of piano and vocals. One of the Viva la Glorias or something.
Witticus: "GeeNef speaks to me like schizophrenia, his words touch me where my priest could reach."