Joo,

In general when you look at an all season tire you will find the tread blocks are smaller with wider/deeper or just plain more spaces between blocks. This is for the snow and rain to "hide" when driving, allowing more contact between a solid surface and the tire. On the summer only tires you will notice wider fewer tread blocks. Snow and rain can't find any place to go so they stay between the solid surface and the tire so you loose traction.

HTH