It actually just means they're using DOCSIS to carry the signal, which has asymmetric bandwidth allocations for upstream and downstream. 600/100 is a standardized allocation too.
In practice, it is always a heavily oversubscribed network that never delivers sustained bandwidth for either up or down.
Contrast to whenever I have used a symmetric fiber connection that advertises 1Gbps/1Gbps, I can actually sustain close to both of those and at sub 5ms latency. Whatever the theoretical promise is, I assume non fiber non symmetric connections are simply low quality (in the USA).
