If I understand correctly it’s not intrinsic to the DOCSIS standards, it’s just how more or less every cable company chooses to allocate channels. Think like a cable company has 100 channels they may be able to use on a given line, and they choose to put 90 of them on download and 10 on upload (numbers are made up to convey idea). Now they have only a small amount of available upload bandwidth and lots of download, but they could have set it up to be 50/50 to have it be equal.