Back before calculators engineers often were less certain of figures than today, so it was not unsual to add 10% (or so) to most figures. To me (today) it seems that 10% is what causes a lot of old buildings to stay standing while a lot of the "enough to get by" buildings of today fail early.
It's likely that there are transient spikes of unexpected stress that rarely happen, but eventually will happen. And that that extra margin that engineers added in the old days was what got buildings and other such things thru the transient spikes of stress without any problems. Take away that margin, and you'll start getting failures from time to time.
