Carriers tell Nokia how many they want, not the other way around. It's quite possible carriers underestimated the demand grossly, particularly on some colors. It's somewhat understandable because L800/900 did not sell like L920 at all and it's not easy to predict how big of demand there will be for L920. I mean, everyone knew the demand would be bigger than before, but how would you know if it will be 3x, 4x or 5x?
Also if I remember correctly, the black model was by far the most popular color last time. This time, red, yellow and cyan seem to be almost as popular which probably caught carriers and Nokia off balance.