not every aerodynamic part creates downforce or "reduces lift" as you call it. some parts just make the car kind of cut thrugh the air more so that there is less wind resistance when you are traveling forward, like what a rear diffuser does and same with those race bumpers with holes in them. try holding a knife under a sink. when the blade is horozontal you will feel some resistance from the flowing water. when you have the blade vertical then you will notices that there is less resistance.

Fun fact: did you know that F1 cars could drive upside-down?