The music is selected based on:
- daytime / nighttime,
- being alone / in a group,
- good weather / bad weather,
- low health / high health,
- world level,
- under ground / over ground,
- in danger / out of danger, and
- near a Titan
.
The aim is to have the music complement your current in game experience.
(But you’re correct that if you largely have the same experience then you will only hear a subset of the music.)