Isn't it that the conical surface is different in size, so one plug might deform the head if you tighten the glow plug hard. Then the other glow plug will have a much smaller area to seal against.
Anyhow, the OS plugs is usually way to hot anyhow, I usually get more tuning issues and a lot less performance with hot glow plugs. Even the Novarossi is on the hot side, but each to his own I guess.