Pi-hole is configured to respond to pi.hole and does not know what pihole is unless you've made your own modifications and changes.
nslookup pi.hole
Server: OPNsense.lan
Address: 192.168.88.2
Name: pi.hole
Address: 192.168.88.5
nslookup pihole
Server: OPNsense.lan
Address: 192.168.88.2
*** OPNsense.lan can't find pihole: Non-existent domain
dan@Viking-1:~$ dig +short pi.hole
192.168.88.5
dan@Viking-1:~$ dig +short pihole
dan@Viking-1:~$