I'm about three weeks into fishing with the LVS34 system and I am puzzled by the distance function when using the system on forward mode. It seems that when I am seeing something that the unit says is 40 feet in front of me the object is really only about 20 feet away. This is very obvious when "looking" at a channel marker that is basically a telephone pole. My unit will show it to be 40 feet away when it is obviously only about 20 feet away. Is this because I am roughly 20 feet of water at the time or is there a calibration that I haven't done?