This is due to how the differential works. Unless it is a locking differential, then power is only applied to one wheel at a time. On 4WD/AWD, the power usually will go to diagonal wheels (ie LR & RF or LR & RF) when their is no resistance. The wheels that spin are the ones with the least amout of resistence. The reason for all of this is when you go around a corner, the outside tires will spin faster than the inside tires. If the differential was locked, one of the wheels would be skidding across the surface.