Certainly the mob aspect was one of Anderson justifications among all the other. Did he not build mines and other land defences against such an assualt?

Second an moving target is much harder to hit than a stationary target tied to a wharf. The job become a whole lot easier when you don't have to adjust for changes in range with every shot and you don't have to cut the time fuses differently.

The southern gunners may not have been that good in January 1861. But practise make perfect.