If they use half arena style, then all seats beside stage and back are not used. I was in first row right in front of Slicky - trust me - the arena was not empty. It was filled to the rafters (don't know if it was sold out or not) but the seats you see were empty because they were not made available for sale (blocked views). If those seats were sold, they'd be staring at the platforms on the side.
It is a strange angle, but the seats in that picture must be on the side.
Love the pictures you're posting though - they're bringing back so many memories - can't believe it was over a year ago now.