Well Steve, experimental evidence and experience always supersedes theory. Being an individual who wants to go into the field of high voltage research upon graduation, I am going to have to play with that software you have. =)
So I guess the real question is, where is the charge going? Since it does not seem logical that a 50% decrease in surface area would yield only a 10% -15% decrease in capacitance. So something else must be going on here.
I suppose that the most logical explanation would be that charge is actually being stored on the interior of the structure for these open geometry designs. Maybe due to the incompleteness of the conductor Faraday's law does not apply with the same rigor as it would in a solid sphere model.