You might hear the same song twice in one night and think the DJ forgot, usually, that's not what happened.
DJs aren't always choosing songs because they're simply “good.” They're watching the room. A track might work perfectly at one moment, then become useful again later when the energy changes.
Sometimes the second version is a remix, edit or completely different version and sometimes the DJ knows something the audience doesn't.
A great DJ isn't necessarily trying to play the most songs. They're trying to control the energy of the room.
People don't remember exactly what was played. They remember how it felt.