50 ohm trace
The simplest answer is that historically, coaxial transmission lines have been 50 ohm, so you design your single ended boards at 50 ohms so that you can go board to board with a standard coaxial cable without big reflections.
However, twisted pair cable comes in more like 100 ohm. It is a function of how many twists per inch the cable has. So you should design your PCB to have 100 ohms differential, so that it will launch into twisted pair with a minimum of reflection.
And, BTW, I do mean 100 ohms differential. ie 100 ohms impedance between the two center conductors of a differeential pair. Not 100 ohms for each trace of the pair to ground!