Subdomain with all digits seems to be messing with /etc/hosts file

I have a /etc/hosts file that has something like this line for 127.0.0.1 for hostnames I'm using for development sites:

127.0.0.1 localhost site1.example.com site2.example.com site3.example.com

I noticed that things broke, however, if I added in a domain with just digits to the entry:

127.0.0.1 localhost site1.example.com site2.example.com site3.example.com 10.example.com site4.example.com

With this line, site4.example.com no longer resolved to 127.0.0.1 but to the IP address for example.com. So it's as if it didn't exist in the /etc/hosts file. The first couple of sites still resolved to 127.0.0.1. But anything placed after the 10.example.com entry no longer resolves to 127.0.0.1. If I remove the 10.example.com entry, it resolves to 127.0.0.1.

Anyone knows why a subdomain with all digits might mess with /etc/hosts?

This is on a Mac.