[Serow225]

Take this with a grain of salt because it's been a while since I've done telecom/telemetry stuff, but even with a Manchester-encoded signal, one of the first steps in the demodulation process is clock recovery from the signal. This usually involves some sort of PLL (phase-locked loop), which has parameters to control it's freq tracking range. However there are trade offs for widening the range, in terms of time-to-lock and increased probability of locking onto noise etc. so for the original application of the system in earth satellites, they probably used a tight range to improve performance.

[jimktrains2]

Yeah, I'm sure there are good reasons and my armchair analysis is lacking, but by filling in that lacking is a good way to learn:)