Why can't my oscilloscope's differential probe display the + and - signals independently?

Differential probes can display the relative voltage or differential signal between their input terminals. Essentially, they convert a differential pair into a single-ended signal and then display it on a single oscilloscope channel.

An important advantage of differential probes over two single-ended probes is that they only take up one oscilloscope channel. Since a differential probe is connected to only one oscilloscope channel, it is impossible for the oscilloscope to display the probe's output signal as two discrete signals.

To see both the “+” and “–” sides of a differential pair, you must use two oscilloscope channels and two oscilloscope probes (or use a two-channel probe).