Minecraft wiki entry for Stairs, associated text: When a stairs' half-block side is adjacent to the side of another stairs, the stairs' full-block side wraps into an "L" shape to join the other stairs (it creates an "inner corner").
When a stairs' full-block side is adjacent to the side of another stairs, the stairs' full-block side shortens to join the other stairs' full-block side (it creates an "outer corner").