General SRO Question

A copule of days ago I went to TKTS to see what was on and Phantom ( a show that I wanted to see) was up there. So, I went over to the Majestic box office and asked if they had any SRO tickets. Judging by the huge number of people waiting to get into the theatre I would have thought that they would. And they did.

My question is this. I was under the impression that they only sell SRO tickets if a show is sold out. Clearly phantom wasn't sold out beause it was up on TKTS. Yet, they were saying at the box office it was and when I saw the theatre it looked full.

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Even if some shows will sell out they still put tickets on TKTS. I saw Drowsy Chaperone closing night and I lost lotto and went over to TKTS. I probably got the last tickets and the show was pretty much sold out, and a lot of people got tickets via TKTS.