I think that those of us who are fairly simple-minded, like me, only need to know that airliners tend to be cruised at the optimum speed for fuel economy, which is usually rather slower than maximum continuous cruise speed. To maintain its assigned flight level at this slower speed calls for a larger angle of attack, achieved by pointing the nose up a bit. So the cabin floor slopes upwards.
(IIRC the VC10 was particularly noted for this, with an optimum economic cruise speed only a few knots higher than the clean stalling speed at cruise altitudes. Cabin crew developed good leg muscles pushing the carts uphill.)