Manufacturers generally test the strength of their magnets using clean and thick pieces of steel. The cleanliness and thickness of the steel used during testing is the main cause for discrepancy between the rated strength and in actual application.
The attractive force weakens very quickly when the magnet is separated from steel. This means that even thin layers of non-magnetic material like paint, dirt or rust can cause a magnet to become less powerful. If you are using a lifting magnet to move something that is painted or dirty, the amount of weight the magnet can hold is significantly reduced. In order for a magnet to be truly effective, it needs to be as close as possible to the magnetic material. This can require cleaning of the surface to remove dirt or paint.
The thicker the magnetic material, the stronger the attractive force. A thin metal sheet will produce lesser magnetic force compared to a thick block of steel.