They were originally designed along the lines of the packs available at the time. So (for CWC, at least) the original base pack channels have the low number, then it's sports and movies, the kids, etc. NTL were along the same lines.
Then NTL decided that from now on it would be a good idea to have the same channel numbers on both platforms for all new channels. So they decided that (almost) all new ones would go in the 800s and 900s. I was always under the impression that this was going to be temporary, until they'd worked out a new numbering scheme, but it increasingly looks permenant
