Originally Posted by HTupolev

People often hit the hills at higher intensity than the flats. Either because they're non-climbers and have no choice, or just because the relative performance boost for increasing power is bigger on climbs than on flats and downhills. And people tend to choose higher cadences at higher power.

On the flip side, some people seemingly also have an easier time delivering high torque in situations with lower crank inertial load (such as climbing), which can result in them choosing lower cadences than would be expected given the power they're doing.