A voltage is relative between two points. A voltage signal therefore always has two connections.
However, a circuit that has to deal with multiple signals usually wants them all referenced to the same point. Since this comes up a lot, we call that point "ground". We then often talk as if a signal is on a single line, implying that it's actually reference to ground.
Your diagram shows Vout between two points, because that's what it really is. But also notice that the bottom Vout point is shown connected to ground. In electrical engineering context, it would therefore be acceptable, and even common, to talk about Vout as if it were only on the top line. When nothing special is said, the default is that ground is the reference for a signal.