For siRNA vs. shRNA, why do different sequences work best in each system?

It is now widely agreed that the rules for rational design of siRNAs and shRNAs are different; however, to date no one knows the answer to the question of *why* they are. Presumably it has something to do with the additional processing steps, such as nuclear export and Dicing, that the shRNAs have to go through before entering the RISC.