Your wrong mate, I'm a toolmaker (injection mould) by trade and they are moulded so the split line is on the diameter of the disc split 50/50, one side of disc is in one half of tool and the other half of disc in other half.
With the plastic being fed via pin gates either on the outer edge or inner edge of center hole. Always surprised me that more discs don't have run lines on them, having said that I think if you removed the labels you would see plenty of them.
Dazz