A theory on why the "ugly banking site" did so well. Out of all of the designs, that was the most traditional, and probably most "comfortable" to the user. The site to me feels like it has depth, and it's friendly to the user, encouraging them to explore. The others while more effective (conversion rate) felt like they were engineered to spoon feed me just the right data, and didn't feel like "complete" sites open to browsing.
This of course is all anecdotal based on my years of being a web geek but sometimes the "feel" of the design speaks volumes where data and raw number crunching cannot.
Perhaps the "Ugly banking site" makes them think you're not a bunch of kids with some VC money ;)
Seriously though, it could be that a less trendy design gives the users some reassurances that you've been around since that design style was in fashion.
This of course is all anecdotal based on my years of being a web geek but sometimes the "feel" of the design speaks volumes where data and raw number crunching cannot.