That's my thought...is that margins at flaps 15 are tight, so 10 knots are added for icing conditions...but not necessary for 30/40 flap landings...
That's the reason. When any aircraft is flying with flap set, it is happy at any speed between the maximum flap operating speed, and a margin above the stall (usually 30% - i.e. 1.3 Vs). Boeing decided to "simplify" this philosophy by recommending manoeuvring speeds, set roughly half way between the two limits, but also adjusted to make them easy to remember and a "logical" progression as you select the different flap stages. (Other aircraft don't have single set speeds, but rely on a bit of pilotage
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The manoeuvring margins are shown in graphical form in the [edit: the flight crew training manual] - and it is here that you can see the flap 15 manoeuvring speed is a bit closer to the low end of the range than the other recommended flap speeds. On the chance that ice has formed on the parts of the airframe not protected by de-ice, increasing the weight (and thus stall speed) the flap 15 manoeuvring margin is the only one that is close enough to the bottom edge of the speed range to worry about increasing it.