She's right. Consider two pumps that can put out the same maximum gallons per hour, but one is a powerful direct drive pump and the other a magnetic drive pump. Start with the mag pump and pump, let's say 1000 gallons per hour into a can filter, but the filter can, let's say, only exhaust 900 gallons per hour. The pressure in the can will rise and feed back through the pipe to the pump. The mag drive will now be overcome by the back-pressure and the impeller will "slip"since it is driven only by a magnetic field. It's output will drop to say 900 gallons per hour, an equilibrium with the output of the filter is established and the pressure in the filter will not increase further. Now, take the more powerful direct drive pump. It starts putting out 1000 gallons per hour into that same filter, the outflow of which is 900 gallons per hour. The pressure rises and feeds back through the pipe to the pump. This time, however, there is nothing to slow the pumps output, so it continues to put out 1000 gallons per hour. Meanwhile, the filter is only getting rid of 900 gallons per hour. Pressure continues to rise until something in the system gives way. In this case, it is likely the O-ring that seals the top of the filter and the excess pressure pushes water out through the seal. Better that it leaks there than to have the pressure continue to build until something else "blows a gasket", such as a water line buried underground. This is overly simplistic an example, but it illustrates the point. That's why there is also a great difference is pumps that put out the same number of gallons per hour, but differ greatly in the "head" they can work against. For example, if you want a pump to put a certain number of gallons of water over a waterfall, you don't simply choose a pump of a certain gph, but also what height to which it can move that water. I hope that helps.
