It is due to the different nature of bending stress versus pure tension or compression. It also is usually only used for ultimate strength calculations and not limit or yield calcs. It takes advantage of the plastic deformation range to re-distribute the bending moment to the inner areas of the structural element being bent, resulting in a nonlinear distribution of the bending moment in the plastic range versus a linear one in the elastic range. It also varies according to the cross sectional shape of the element being bent. It results in the "apparent" allowable bending stress to be higher than you would expect. In pure tension or compression, the entire cross section has about the same level of stress, so there is no place to re-distribute the load to. I beam sections do no benefit much from this effect, and a diamond shape is the best since it has more material in the central area where stress is initially low, and therefore can re-distribute alot of load there. Look up plastic bending in a textbook or manual.