In those species that rely on venom to kill their prey, the venom is evolved to deal effectively with the specific prey the animal lives on. Since scorpions do not prey heavily on humans, any effect the venom has on humans is incidental. However, there are many scorpions that rely more on their claws for prey capture and not on their venom. In these the venom MAY be used primarily for defense, in which case it may be more effective against primary predators (such as mammals). And in some species the venom may not be particularly important for either prey capture nor defense - just a relic of their ancestry that may be slowly on its way out.

I guess this is a long-winded way of saying there isn't really a correlation between venom effect on humans and venom efficiency on prey.