because a -96db sine wave is represented with exactly the same value on a 96/24 and 96/16 representation. The same is true with a -80db sine wave and a -24db sine wave. A -97db sine wave isn't able to be represented by a 96/16 representation, but is on a 96/24.
The image you're showing represents sample rate. But nothing reproduces music with the stairsteps anyway, the sine wave is then interpolated and generated as an actual sine wave, as well.