To be fair, 90% confidence means 90% on average. From one test like this, I'm not sure you could conclude much difference in ability to estimate or synthesize confidence levels between people who score 8, 9, and 10. Indeed, because of the gaming ability for picking 9 with -inf to inf bounds and one with tight bounds to force a 9, I would weight a 10 achieved with tighter bounds as better at confidence estimation as a 9 achieved with wildly different or generally wider confidence bounds.