2slow: Pilot error is replicated with the random swing. Imagine a meter. If it's centered the crosshairs are calibrated. Either side means the bombs fall short or long. When you hold the calibration key the needle swings wildly back and forth. Depending on how long you hold it, you could be 20mph off and drop a friggin' mile off target.
The longer you hold it the shorter the "swings" of the needle become, and more likely it will be "calibrated" to your current speed.
There is error built into the system. It takes a little more time to avoid this error, but so would the proper in-flight calibration in real life.
I think it's a fine compromise, but perhaps a bit random. I'd rather have second and third calibrations (after you calibrate within 5mph of current speed) to refine the existing calibration, but instead every time you re-start it, the needle swings wide again.