I have noticed the same. Is it the roll resistance from the slush/snow that helps achieving a better deceleration if stopping, hence you can take more weight? That's the only reason I've come up with so far.
Actually, it's just the opposite. On a contaminated runway, you will most often be limited by the the "stopping case", i.e. you will require more runway to stop for a given V1. To satisfy the requirement to stop after an RTO at V1, you begin to reduce the V1 value with the limitation that V1 must be greater than Vmcg and Vma. With an assumed temperature takeoff, all rated power remains available and thus, Vmcg and Vma are predicated on full thrust. As has been highlighted before, a derated engine is akin to "buying an engine" with lower thrust so the Vmcg and Vma values will be based on that lower thrust and will be lower, allowing a lower V1.
I would be surprised if Airbus didn't have the same restriction on use of assumed temperature on contaminated runways though I suppose it would be possible to reduce the thrust to the point that you still maintain the full rated thrust Vmcg and Vma. If that is so, you would be denying yourself some of the value of the increased payload that you sometimes get when reducing the Vmcg and Vma values.
The really important point that I don't think is emphasized enough is that when using a derate, you risk a loss of control when pushing up the power beyond the derate limit in an asymmetrical situation. When using a derate WITH an assumed temperature, you theoretically have additional power available to you up to the derate limit but unless you actually know what that limit is, you're playing with fire. Most carriers that I'm familiar with do not have procedures for calculating the actual derate limit N1/EPR/TPR, etc..