That's an arp table maintained withing pfsense. It's not the arp cache as used by freeBSD. Go to a command prompt and enter the command "arp -a", which will show the contents of the arp cache. If a host name is there, it's because something did a host lookup on the IP address. An arp cache is used when trying to reach an IP address on the local net, by providing the MAC address for it. A host name has nothing to do with that function.
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