There is not really enough information to go on here but the most obvious thing that stands out is the router and firewall IPs ie.
192.168.100.1 255.255.255.240 = subnet 192.168.100.0, broadcast 192.168.100.15
192.168.100.16 255.255.255.240 is actually the subnet address eg. subnet 192.168.100.16, broadcast 192.168.100.31
if those IPs are right then the router and firewall are not in the same subnet.
So when the router wants to send a packet to the firewall it knows it does not have a directly connected interface in the destination IP subnet so it looks in it's routing table for a matching route.
Can't really say much more without more details ie. would need to see routing tables of the router and the firewall.
Jon