Caster sugar/castor sugar

Both spellings caster and castor are acceptable with the OED favouring castor, but why is it called castor sugar? For the simple reason that the sugar is fine enough to be dispensed from a castor or caster, which is a small vessel with a perforated top from which ground salt, pepper or sugar etc can be cast. Today, we would be more likely to refer to such vessels as salt or pepper cellars but back in the late 17th century everyone referred to them as castors or sets of castors.