I think that once the bushing is toast, the weight and its mass can be just a bit further out, which would cause centrifugal advance at a lower rpm?
also a rusty spring isn't going to have the same spring rate
You can also fire a timing flash with it connected to #6 because that is also on the flywheel mark. That might tell you about the overall precision of the distributor if it is the same as when firing on #1, or not.