I don't think the subnet mask is wrong. If the router/gateway address is 10.10.39.1, and the netmask address is 255.255.255.0, then all the addresses between 10.10.39.1 - 10.10.39.254 belong to the same subnet and that's OK.
255.0.0.0 is for 10.x.x.x, not 10.10.39.x! With 255.0.0.0 the local subnet (/8) is 10.0.0.1 - 10.254.254.254. Maybe too much, of course it all depends on the network design. However, in the screenshot above the router address is correctly located within the /24 subnet, so the netmask is correct.