I think I understand now. By default in pfSense the reserved/static mapped IPs are not part of the DHCP pool, but are handed out by DHCP. For instance, a pool might be 192.168.0.11 - 192.168.0.100 and you would have a static mapping for a MAC of A1:B2:C3:D4:E5:F6 to the IP 192.168.0.10 so that device gets that IP via DHCP, but that IP is not part of the pool. This is different from say a Windows DHCP server, which will hand out a reserved IP from within the pool.