well quick test to make sure its pfsense or not, unplug pfsense lan from your network ;) Does it still get answered? If your showing an answer from that mac, then it would be in your clients arp table if on the same layer 2.
But pfsense might be sending it out its wan, and something upstream could be answering. If that is the case then yeah you would show mac of pfsense lan as the answering mac..
That would be my guess to what is happening.
perfect example of this is me pinging my cable modem management IP
ping 192.168.100.1
Pinging 192.168.100.1 with 32 bytes of data:
Reply from 192.168.100.1: bytes=32 time=26ms TTL=63
Reply from 192.168.100.1: bytes=32 time=1ms TTL=63
Reply from 192.168.100.1: bytes=32 time<1ms TTL=63
Reply from 192.168.100.1: bytes=32 time=1ms TTL=63
my pfsense wan is public.. But I can still access my cable modem via that rfc1918 address since pfsense wan is directly connected to it. If something on your wan answering - sniff on pfsense wan and find the mac that is answering. It might be showing your gateway on your that network, but then you would know its something else upstream.