It was assumed that under 100 were admin / service accounts, ie: not real people.
One aspect of that ended up that the useradd command started at 100.
Which in turn meant I got used to my id being 100 on my systems, and was quite annoyed when I ended up being the 2nd person added to certain systems at work.
I ended up swapping IDs with the admins to the NFS would work without setting up maps.
This was local passwd file access of course.
Later Linux useradd started at 500.