I remember seeing a series of television programmes on the 777, in one of which they showed the wing tested to destruction by application of hydraulic rams across the surface. From memory, the test criteria was 1.5 x design load, so this is a standard number. The load was applied slowly.
I would suggest the issue of tolerances, live versus static loads, accuracy of computer models etc. is the reason for the 1.5 factor. This is a considerably higher factor of safety than is applied to other structures which are not tested (such as support structures for deepwater oil production platforms).