Satellite uplink/downlink frequencies
The main reason the uplink and downlink signals are separated is because you don't want to mix high power and low power signals in the same band. The LNA on the input can be saturated by out of band signals that are too close to be filtered out. On the ground, the uplink will be extremely powerful while the downlink will be extremely weak, so to have any hope of receiving the downlink you have to really separate the frequencies so you can filter out the uplink effectively. Now, specifically which frequencies are used is a convention, but I imagine that the uplink frequencies probably tend to be higher because ground stations generally have a lot more power available to run the transmitter than the satellite and so can work more effectively with higher frequencies that have more path loss.