Hi Jirong,
Your power meter probably uses a data type called CTF or crank torque frequency which you can confirm using the Plugin Sampler. CTF meters use a value called the zero offset to influence the power calculation done by the display. Typically a CTF power meter automatically transmits a new value while it is coasting, but it sounds like the Plugin Sampler may have received or was using a different cached value from the one your 910XT was using. By sending a manual calibration command, you force the power meter to send a new calibration value, which is now the same one your 910XT is using. I believe SRM recommends doing a manual calibration once per day at the start of the day to account for temperature changes and the sort. The plugin sampler does not do this automatically, but it does store the value between the times it receives a new value.
Best regards,
Harrison