Yeah so the water flow or volume of water through is dictated by the pump, and the refugium has the same volume of water moving through it but the speed is much slower.
The water velocity in that area is slow. That's because the refugium area has like a water fall into it that is much WIDER than the drain pipe. So to get 1200gph through the refugium it doesn't need to flow as fast through the refugium as it does the drain pipe (physics stuff if that interests you).