The Kodak CCD in the M9 is the best they could do at the time, the sensor is now over 2 years old. Advancements in sensor technology make it credible that the M10's sensor could be an improvement in several areas including ISO noise, dynamic range, and even megapixels.
More megapixels does not always mean more noise... For example, look at the original Canon 5D vs. Canon 5D mk II... The mk II has 9 more megapixels and 2 stops better ISO noise. Another example is the Canon 1D mk4 vs. mk3, same story, the mk4 has more megapixels and lower noise.
I'm not saying 18 mp is bad, I'm just saying there is nothing wrong with having more if anything for cropping power. With prime lenses the extra megapixels are often a god send. Heck I prefer using my M8 over my 5D2, but that doesn't mean it wouldn't be useful having extra megapixels to play with.