I occasionally moderate QF. I do not admit to being consistent. Sometimes I give people the benefit of the doubt and leave their question open, other-times I internally chastise them and vote close without a second thought, and I occasionally form a coherent thought process and make a decision. I only detail this since I suspect other moderators are on the whole more or less similar.
Coupled with a potentially small pool of active moderators is the requirement of 5 votes to close a high bar? Indeed, I have no statistics at all and might be wholly mistaken, but I suspect if someone were to produce a (should have been closed / actually closed) ratio it may be quite low whereas an (almost closed/ actually closed) might be significantly better.
I suppose it raises a second point, which is if the goal of the site is to attract more high quality Q&A, should be doing a better job of separating the wheat from the chaff? I don't know the psychology of social network aggregation so I can't help out with this one either.