I mean I was able to put those two subnet masks in, and I was able to ping that IP address and do web hosting off of them. I agree, that it seems odd that both 255.255.255.0 and 255.255.255.240 work. And no, I only have one default gateway set up as 18.104.22.168, but I don't know what it's subnet mask is.
They would both work because ideally as long as you only used IPs within your assigned subnet it 'should' be switched/routed properly (ymmv there). If you tried to use IP'd below your own assigned subnet (as it looks like you have the tail end of the /24 in question) then you would start seeing things begin to break.