The disappearing cup and book are caused by a similar issue. They "only show detection results with confidence value that exceeds 0.5". The cup is shown at 50%

The disappearing cup and book are caused by a similar issue. They "only show detection results with confidence value that exceeds 0.5". The cup is shown at 50% in the original image, so it "disappears" if the presence of the elephant lowers its score even slightly. Same of disappearing books. Presumably the couch showed up because it rose from <0.5 to 0.57. They don't tell us what the score is for the chair, it might even be higher than the score for the couch, since their visualization clearly doesn't sort by probability.