Classic vintage detail. 

By adding the pocket horizontally (with the weave going sideways - hence hiding the selvedge), against the rest of the denim (with a vertical weave), the denim achieves a unique quality and wear. The pocket shrinks separately from the rest of the denim due to the opposing weave. This quality is sought after by many denim and vintage afficiandos. 

Take a look at the different ways these two pockets have shrunken. In Japan, some call the left wear the "Inazuma Line" or "Lighting Line".