By default, nmap is set to sometimes resolve an IP address to hostname and uses reverse DNS (meaning that an IP address is resolved to a hostname) lookup. Nmap uses a custom stub resolver, not your primary or secondary DNS servers.

In this example, the -sn option is used to determine if a host (e.g. a target system) is up may return something like this. In this example, IP 192.168.0.16 is resolved to hostname server1.example.com and 192.168.0.17 is not resolved to a hostname.

nmap -sn 192.168.0.0/24

Nmap scan report for server1.example.com (192.168.0.16)
Host is up (0.00063s latency).
Nmap scan report for 192.168.0.17
Host is up (0.00059s latency).

The -R flag can be used to always resolve an IP address to hostname.

nmap -sn 192.168.0.0/24

Nmap scan report for server1.example.com (192.168.0.16)
Host is up (0.00063s latency).
Nmap scan report for server2.example.com (192.168.0.17)
Host is up (0.00059s latency).

The -n flag can be used to never resolve an IP address to hostname.

nmap -sn -n 192.168.0.0/24

Nmap scan report for 192.168.0.16
Host is up (0.00063s latency).
Nmap scan report for 192.168.0.17
Host is up (0.00059s latency).