Some nameservers deliberately return IPs for nonexistent domains. ISPs are notorious for doing this - they can actually monetize on advertising given on landing pages for nonexistent domains.
You could always change your resolv.conf file to use public DNS servers that are known for sure not to exhibit this behavior. Google‘s DNS (8.8.8.8 and 8.8.4.4) and Level3‘s DNS (4.2.2.1 through 4.2.2.6) both provide public DNS access and do not redirect unknown domains. (Source: https://www.grc.com/dns/alternatives.htm)
