Graham.
When fitting skirting boards (wooden at least, but good for MDF etc), on an outside corner you do a 45° mitre joint. As the wood does not shrink along the grain the corner stays tight.
For an internal corner if you did the same the joint opens up. So on an internal corner one goes straight into the corner (it's best if it's on the wall most looked at, so one opposite the door as you walk in the wall either side of fireplace etc). This is because any shrinkage will show up less. So we have one board into the corner, the board on the other wall has the end shaped to fit the shape of the already there. When fitted together you see only a line. As the wood shrinks across the grain the only one that will affect the corner is the one that goes right into the corner, you in effect halve the damage shrinkage can do.
Back to the order the boards get fitted. If the board goes into the corner of the wall you normally look at, you along the shaped one which has the effect of hiding any shrinkage. Now the good thing is as you go around the room you always or nearly always have one end shaped and the other end plain, makes it so much easier to fit.
I can do a fag packet sketch, if need. Like most things easier to do than describe.