At a total guess it's down to the lack of precision in the output at anything other than full power. Most (all?) microwaves just pulse the magnetron to achieve

At a total guess it's down to the lack of precision in the output at anything other than full power. Most (all?) microwaves just pulse the magnetron to achieve an average delivery of the rated power so the difference between 400W and 425W would be dependent on the time the microwave runs, given most tend to use a fairly long time period. You can see this effect if you run a microwave on low power for a short period - sometimes the magnetron will be running for the full duration.