That link's explanation pretty much sums it up.
I have long wondered why the difference in the factor is safety
for HT. My best guess is that a bigger difference in SWL was needed to justify the cost difference and that this was rationalized by increased inspection
of the links after proof testing, 100% instead of 5% are checked for elongation or other types of failure. I don't know how this works with higher grades. I suspect the reason for the difference is lost
though the folks at Acco might know.