Why is the primary admin UID 501?

I understand* the primary admin user is given a user ID of 501 and subsequent users get incremental numbers (502, 503, …). But why 501? What’s special about 50x, what’s the historical/technical reason for this choice? \* I started looking into this when I got curious as to why my external hard drive had all its trashed files inside .Trashes/501. My search led me to the conclusion 501 is the user ID for the primary admin in *nix systems (I am on macOS), but not *why*.