I can't say this is 100% correct but it seems to me that the more contaminated a runway is the more slipery it will be. If this is true then the traction would also be less when it comes to controling an engine out scenerio. Because if this the rudder must be able to compensate and this is done by giving you a higher speed, hence a higher weight. Of course this only comes into play if there are no other limiting performance factors.
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