Help reading transmission level?

my boyfriend just changed our transmission fluid and well here is the problem. when ever it he changed it he checked it and it was fine but this morning after sitting all night he goes to check it and it says it overfull so he starts it and lets it warm up and checks it and its back way down to the warm mark it (says 20c) on the stick then after the long drive to work he checks it and it reads on the hot mark (175f). what is with the different readings. he keeps trying to tell me that when it sits all night the fluid in the torque converter goes back to the pan or something