think about a piston moving in a piston chamber. Piston moves only up and down the engine block. But piston is linked to rod with a rotation axis, and the rod to the crank shaft also with rotation.
If you ask for swept volume of piston with regard to block (fixed) you have the full displacement of the piston in the engine.
But if you ask the swept volume of the piston with regard to the rod, you will see the piston rotation on its axis.
Eric N. indocti discant et ament meminisse periti