The drag coefficient of a bike rider is quite variable, and nothing like a constant. My drag coefficient varies according to whether I'm sitting normally, tucked down, sitting up or standing up. It also varies according to the clothes I'm wearing, as a baggy coat easily adds 10% to my drag. When I stand up and really stamp on the pedals, the bike weaves and the drag coefficient fluctuates very quickly.

As others have mentioned, a head/side/tail wind makes a huge difference. You could try mounting a wind speed sensor somewhere on the bike, but it might be quite hard to find a mounting position that gives an accurate reading.