Why are capacitors sold with imbalanced tolerances?
Unlike resistors, whose price is essentially independent of resistance except at extreme values which represent less than 0.01% of the market, most types of capacitors have a cost which is tied strongly to capacitance--it costs more to make a large cap than a small one.  Further, capacitors are often used in circumstances were a cap which is larger than specified might work better than the specified one, up to a certain limit, but the bigger cap might not be worth a higher price.

Suppose a designer determines that a device needs a minimum of 8uF to work correctly in a particular situation, but anything up to 20uF would work just as well.  Some manufacturers can produce devices within +/-20% of their target; other manufacturers are capable of +/-33% of their target.  If published tolerances were symmetrical, one would have to specify that the part could be either a 10uF+/-20% or a 12uF+/-33%--a bit awkward.  If, however, manufacturers by convention use -20% for the lower tolerance and adjust the upper tolerance as needed, then it's possible to directly compare and substitute parts with different tolerances without affecting circuit operation.