Hmm....I would venture guess here and say that 'general' med manufacturers such as API or Tetra have some wiggle room (so to speak) in their dosing suggestions as most are quite under-dosed (antibiotics) and/or have a margin of error (ich meds). Medicated foods have limited wiggle room but really should be fed appropriately to ensure they are effective (dose via weight). Purchasing pure meds in bulk from a manufacturer or using non-standard aquatic meds (such as injectables) do not have much leeway for error and typically do not come with specific (if any) instructions.
Quite a bit of all this really is dependent on the type of med being used, fish being treated, tank conditions (some meds do not work in high ph or are made toxic, opposite for some others in acidic water), etc. Which makes all of this more complicated, unfortunately. In my opinion and going back to the original point, I do think general manufacturers incorporate some amount wiggle room for hobbyists or there would likely be a lot more casualties being discussed. Not sure if this helps?