There are a remarkable number of variables at play when it comes to choosing the right magnet. One such variable is the orientation of the metal surface you are adhering magnets to.
The Pull Force listed for each magnet is based on lifting 10mm thick steel vertically. Magnets on a vertical surface (of 10mm thick steel) are generally able to hold around 30% of the pull force listed due to the effects of gravity and the lack of traction between the surface and the shiny magnet.
An example of this 'rule' is a magnet that has been tested to a pull force of 1kg (testing is conducted with 10mm thick steel lifted/holding vertically). When placed on a vertical surface the pull force is diminished to approximately 300grams.
Fridges and metal lockers are normally constructed with 1mm or 2mm thick steel, which is significantly thinner than the 10mm steel the magnets are tested against and the magnetic field is not able to achieve full saturation of the steel and therefore its 'full' holding power.
Also, any gap created (such as paper) between the magnet and the metal it is magnetically adhered to will also slightly diminish the holding strength of the magnet.
We do our best to assist customers with finding the best magnets for their application however it often requires some trial and error to select the correct magnet for your project.