My guess is that some of the winter tires generate optimum traction in deep snow when there is some slippage, due to the tread design, which allows the tire to dig into the snow, generating traction as it does. This slippage is prevented however, by traction control.

The same argument goes for a locked tire generating more traction in deep snow than one that is ABS-modulated: the locked tire is able to dig into the snow, generating more stopping force, due to the dam of snow that forms in front of the tire. This same principle is used for the sand/gravel traps in the runoff areas of race circuits.

- just my Cdn$0.02 ...