@Troye-Johnson My intuition is telling me that you have a second dhcp server responding to this request.
If your fog server and pxe booting target computer on the same subnet? If so lets grab a pcap file of the pxe booting process. That will tell us what is really moving down your wires. I have a tutorial here that describes the process. https://forums.fogproject.org/topic/9673/when-dhcp-pxe-booting-process-goes-bad-and-you-have-no-clue
Post the pcap to a google drive or dropbox and either IM me the link or post the link here and I will look at the pcap and tell you what I see.