Where does that hiss on the master come from?

I was recently listening to a remastered version of physical graffiti and I was amazed at the amount of noise it had on it - I mean its remastered! Then I listened to Aja which has absolutely no noise in the back. I asked a very knowledgable friend why the remastering couldn't get out all that noise and he said that the noise must be in the master. My question is how does noise get on the master? Thank You!