Q.: Why don't we get grooves and ridges of reasonably uniform width and depth when we broom a slab for slip resistance?

A.: It's not too much of a trick. Keep a wide, shallow pan of water at the edge of the slab. Before each pass, dip the broom in the water to wet it and remove any mortar clinging to the bristles. Be sure to tap to remove excess water, then use the broom.