Why would a propeller have blades of different lengths?
You are right.  They are unequal length blades.  Schempp refers to the propeller as being "noise optimized" and you can see that the blades are free to flap (forward) like helicopter blades (which flap up) so that the asymmetric thrust axis from having the propeller disc offset from the rotational axis doesn't place bending loads on the hub.  And obviously, the blades will be mass balanced to place the center of gravity at the hub axis.

I'll go out on a limb here and say that some genius figured out a way to reduce the horrific noise level caused by such a small propeller by having the blade tips each run in a slightly different path of rotation.  Brilliant.



And here it is in action where the eccentric blade disc can be clearly seen.