That is exactly the point. The speedometer reads the same as the front wheel continues at the same speed. But a taller rear tire reduces your RPM's at the same speed, or conversely raises your speed in relation to RPM and front tire rotation.
Mine read the same error rate and at top speed it is about 9 mph high. BTW, with the 55 tire mine is still off 1 mph at 70.
I'm not sure if I follow what you are saying. A taller (or shorter) rear tire will affect the relationship of rpm's and actual speed, regardless of the speedo drive pickup.
But ....
1 - The C14's speedo is not driven off the front wheel.
2 - On a bike with the speedo driven off the front wheel, changing the rear wheel size (or gear ratio/sprocket sizes) will have no effect on the displayed speed.
3 - On a bike with a rear-drive speedo, changing the tire or sprocket sizes will affect the displayed speed.