Your logic is incorrect. That the driver was at some point going faster does not negate that the same driver was at some point going at a lesser but still illegal speed. In that case, charging for the lesser but known speed is still a proper and accurate charge.
Cops are permitted to lay charges for less than the actual observed offence. They do it all the time.“I find the reasoning of the Court in Nicholas v. Penny highly persuasive and I am prepared to follow it. In short I would hold where evidence is given that over a measured level distance, the speedometer recorded steadily at 95 miles per hour, this is prima facie evidence that the offence of driving at 90 miles per hour was committed, in the absence of some evidence, elicited either on cross-examination or by defence witnesses which would suggest that the speedometer on the police vehicle was inaccurate.”