First possible (yes, rational) explanation is that she’s got her legs sideways on the couch and her friends are just sitting atop them. The other idea is that she is sitting with her legs over the couch like everybody else in the picture, you just can’t see them. Look closer at the one bare kneecap on the left. That’s actually the knee of the “legless” woman blending in perfectly with the all-black legs of the woman on her right. If you need more proof, note that the bare ankle on that “leg” stops halfway across because it’s actually two legs and not one.
Unfortunately, some genius had to go and ruin the fun, pointing out that both girls on the far left are each wearing black pants, making all four of their legs camouflaged and kind of blend together in an amorphous blur, hidden behind the woman on the far left’s. A sliver of ankle seems to prove this theory.
Do you have an interesting theory for us?