Michael,
There's a lengthy thread somewhere on this subject with many diverse views including just whether sat. nav. readings are themselves reliable.
If you have a message centre in your instrument pack, a digital speed display can be activated by T4. This will give you the more "accurate" speed you desire. Since the same signal drives both this digital display and the analogue dial, its likely that the error was deliberate as pab suggests.
Simon
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"Whatever is rightly done, however humble, is noble."
Sir Henry Royce.
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