Some thoughts -
(a)
herein lies the inequity It would be totally impractical to restrict real world occupants to the design loads - but we can do that relatively easily for baggage and cargo. As an anecdote similar to yours, many years ago, a freight MU2 equipped with my design restraint system pranged with predictable results. The investigator gave me a call to let me know that the aircraft had disintegrated around my restraint system - allowed me to sleep easily that night .....
(b) one needs to keep in mind that, while the heavier folks end up with a "compromised" seat structure, the lighter might end up with more injuries due to higher accelerations, especially in dynamic seat installation helicopters where the download is spine-critical. Guess we just have to accept that there are no guarantees, only probabilities, swings and roundabouts.
(c) Some years ago, John Klingberg (then an Australian regulator engineer) prepared a very useful report which (eventually) formed the basis for the present protocol -
https://www.casa.gov.au/files/2351pdf
John's report makes a good story for a sensible statistical approach to loading. If you can locate a copy, it makes interesting reading. Mine is someone in one of the older filing cabinets, gathering dust.
(d) As double_barrel suggests, the probability of misloading using standard weights is very much tied up with total numbers, as shown in John's report. Interestingly, in a previous life flying F27s, periodically we would have to weigh the load when fuel was really critical and we were after every last drop. With the typical 30-40 pax load, 170lb worked out real fine. That figure originally came from a study of North American Army personnel back in the 40s as I recall. I have the report somewhere ... As Dave indicates, one is better off weighing small loads ...