pitted front discs

[pomm001]

My 2002 ADV has pitting on both front disc, its done 21k miles, its isolated to a couple of spots,the brakes work fine, and i am not going to replace them just yet. But I am curious if this is common, i have not seen pitting like this before on a brake disc, it resembles damage similar to cavitation, small holes in the surface metal of approx 1mm deep,by about the same wide, I dont think it is from surface corrosion, any ideas ?
the rear one is OK

 

[Paul Young]

If you are a "demon late braker," it could be the heat precipitating the carbon out of the steel. Not a major issue, but a little unsightly.