These attributes change in response to events in the game. When a civilization would reach the modern era, it's aggression attribute would be reduced by 2. Gandhi's aggression was reduced from 1 to 255 (underflow). He would rain down nukes on everything in sight, somewhat uncharacteristic of him.
Fans liked this so much that Firaxis maintained the same behaviour in later games.
Especially back in the 1980s and 1990s you'd get awful code that did things like averaging the two streams because wrapping sounds awful and the authors were ignorant of the theory and/or unaware that saturated addition is a thing.
You can tell when somebody did this because it means playing silence makes everything else quieter, or worse there's an arbitrary limit on how many streams are played and playing any one thing is very quiet because it's attenuated so as to never wrap.
Haiku the 1990s-style operating system did this for years, as did various Amiga music software.
