well theres a number of other factors that will also alter the freeze point obviously
age of the coolant
other coolants still in the system
actual (exact) % of the mix
was the system flushed with water before it ws refilled (poss water still in the system)
other additives that might have been used (stop leak, ect)
contamination in the system
system pressure at the time
but my original point still stands, extra antifreeze does NOT add extra protection after a certian point