[mrtexas]

Likely set up that way so the fan doesn't get harmonic vibrations. With the blades spread evenly and the same length there is the possibility to get vibrations that get bigger and bigger that could break off the blades. If you look at the blades of a late model car they can be even more un-even to prevent the destructive vibrations. The analogy is pushing a kid on a swing. If you push at just the right time the kid goes higher and higher.