You might find one is actually static for server hosting purposes like web & email, remote access/vpn etc, the other if in a different range is variable so you can surf the web with an element of privacy, at least thats what UK ISP's do anyway, but as I also discovered when ISP's hand out an IP block when ordering a single static ip, the ISP have in fact given all the ip's in the block. Whether your ISP have done this if its so large, only you can find out by setting up pfsense to accept incoming on the other ip's in that block, or they may have some other setup upstream to restrict you to one ip.
Have a poke a round if interested in finding out.