If you were to remove the barrel you would find there is a slightly raised round shoulder on the barrel that bears on the frame face. It is a few thousandths high and the area around it slightly relieved so as to not drag on the frame face and mar it when the barrel is tightened. In other words, it is deliberately done to prevent cosmetic damage during assembly. Since this is not a critical dimension some are relieved more than others. If you look close enough virtually every revolver with an extractor rod shroud will be found to have this gap, some bigger than others.
Photographing it with a bright light behind the gap exaggerates it considerably.