Like your thinking
Suppose the fish are feeding on a bed of particles, and you put a pop-up out there. The pop-up level may actually be above the fishes feeding, and so ignored. The fish feeding in the silt, noses in, pop-up is above the carps feeding level, and again, ignored
Or a pop-up at 2cms off the lakebed, a big fish approaching from above might not even recognise that the bait is off the bottom.
My view with pop-ups, you need to know what type of silt, or lakebed you are fishing over, and to some extent, the depth of the silt.
No point fishing a pop-up with the weight 2cms under the hook if the silt is 3cms deep, and smelly, thick and black. You need the pop-up to be above the silt and bottom debris.
Also no good fishing a pop-up above the level the fish are feeding at.