Pull force, shear, and why a heavy notice needs a strong magnet, not a stronger wall
The number on a magnet's packet is measured in a lab condition that a wall almost never reproduces. Here is what actually holds a sheet of paper up.
Magnets are rated by pull force, the load it takes to pull the magnet straight off a thick, flat, clean piece of steel. It is a useful number for comparing magnets, but it is a best case. Two things on a real wall knock it down hard.
The first is the gap. The field of a small magnet falls off roughly with the cube of distance, so the force drops steeply the moment anything sits between the magnet and the steel. On a magnetic-receptive wall the steel sits under a decorative face and often under paint, and that facing is a gap. A magnet rated for a heavy pull on bare steel may hold only a fraction of that once it is working through a finished surface. This is why the wall's build-up matters: the thinner and more consistent the layer over the steel, the closer the magnet gets and the stronger the grip.
The second is direction. Pull force is measured pulling away from the wall, but a notice pinned to a vertical wall is not trying to pull away. It is trying to slide down under its own weight, and that is shear. A magnet resists sliding far more weakly than it resists a straight pull, so a magnet that could theoretically lift a pound off a bench may let a few sheets of paper creep downward. The fix is not a stronger wall, because the wall's steel is already continuous; it is a magnet with more contact area or more strength, or spreading the load across several magnets.
For everyday planning, treat the rated pull force as a ceiling you will never reach on a finished wall, size the magnets for the shear load rather than the straight pull, and test with the actual finish in place before assuming a display will stay up.