The reason it shows 2 devices is probably just because the router keeps a history of devices that have been connected to it in the past. If your router shows a list of ‘active clients’ there should be only one OpenSprinkler device there.
You are right that the device ID sets the last byte of the MAC address. This is provided as an option for those who want to use more than one OpenSprinkler on their network. Normally if you just have one OpenSprinkler you can leave the device ID as the default 0.