I think that in some of the cases where someone shoots DA more accurately than SA (or, at least, almost as accurately), the reason is that the shooter has never conquered the flinch. If you are prone to flinch, it will have a bigger effect when shooting SA than when shooting DA, because of the long trigger movement on DA before the hammer falls. If you've conquered your flinch, SA is MUCH more accurate than DA (especially if you've got a light SA trigger).