Pump head is not only the height from the water surface to the outlet. In a real drainage or wastewater system, pipe length, pipe diameter, elbows, valves, and outlet pressure all add resistance. If these details are ignored, a pump may look correct on paper but deliver less water after installation. This is a common reason why some sites see slow drainage even when the motor power seems enough.
A clearer way is to draw the pipe route before selecting the pump. Mark the vertical lift, horizontal distance, elbow quantity, valve type, and expected flow. Then compare these details with the pump curve. For dirty water, the model also needs enough solids passage, because a narrow passage may block even if the head is enough. Checking head and pipe layout together makes the final selection more practical and reduces changes after the pump arrives at the site.