If the fork cross section is less than the larger (longer forks) that may be the difference. In additions the "chemistry" of the forks metal may cause a reduced capacity (i.e. low or high carbon steel) - this can change from fork manufacturer to manufacturer How forges are made (fully forged or other wise) can make a difference as well. In a nutshell, not all forks are a like and they can be the weakest link that determine the capacity rating.