Sure, the same gear in the t-case spins the speedo cable, but once it enters the back of the speedo there are two separate devices there.

One is the gear driven counter (odometer) that cranks over 1 mile for some many revolutions, essentially a digital machine.

The other thing is the speedometer that is analog meter with a spinning magnet inside an aluminum cup that the needle sits on and a spring provides the return force. This is not terribly accurate and over time can change calibration. You can have the speedo recalibrated to read closer, but I think most speedos (at least the old magnetic kind) are lucky to be +/- 5% accurate, some are worse.