Magnet variation comprises physical dimensions as well as material composition. This design used a block shaped magnet and the relevant dimensions include magnet length, width, and thickness. Magnet composition can vary due to impurities, and the variation in composition corresponds to a range in remanence, Br, which directly relates to field strength.
The B221 magnet used in this document has a tolerance of ±0.004 inches for all dimensions. A typical N42 magnet like the B221 has Br of 13000 Gauss. Product specifications outlined a Br maximum of 13200 Gauss, thereby suggesting a possible tolerance of ±200 Gauss.