Evenly spaced blades, symetrical, tend to noise from the even "pulses" as the fan blades hit the airstream through the radiator. The 5 blade fans could be manufactured with the blades equal spaced, but that would not accomplish a desired result. Three are moved slightly closer together and the two remaining are noticably closer on the opposite side. This maintains static and dynamic balance and breaks up the frequency of the air pulses, reducing fan induced noise.