Yes, exactly that. We don't have the luxury of many events, so to increase our confidence we generate more background. One of the problems is what to do with da

Yes, exactly that. We don't have the luxury of many events, so to increase our confidence we generate more background. One of the problems is what to do with data that contains more than one event. In the second detection paper, one of the plots contains the background with and without the other event taken into account. This is because during the time sliding process you will necessarily slide the time series from one detector across a gravitational wave signal in the other, producing false background noise. There's no real standard way of dealing with this, so we show both cases - but both claim 5+ sigma.