[saalweachter]

I would note the big difference in that trial is for summer vs snow, with all-season showing smaller degradation.

So there's two things going on: first, snow tires are a softer rubber that functions better at lower temperatures. There is a zone of "warm" snowy weather where all seasons are just fine, but then you get below 20F or 15F or so, and the all season start to harden, and their performance drops off a cliff.

Second, snow tires are made to get packed with snow at low speeds, because the friction of snow on snow is higher than snow on rubber.

So damn cold weather or enough snow for the packing to kick in, and two snow tires will start beating four all season.