The equations we use to discribe fluid flow are mostly approximations. The equation for maximum speed for a displacement hull is really just a fancy curve fit. It works for basic hull shapes and if the ratios of the length, width and depth are within limits.

If you make the hull narrower, you have to make it deeper to support the same weight. At some point the hull gets deep enough that maximum hull speed equation is not valid anymore.

I know this explaination is not very satisfying but the real explaination takes about 3 chapters in a book on Fluid Dynamics and they don't even talk about it directly.

Bethwaite's book is a good top level start but he he doesn't go into the details that would matter if you started building something really strange like a hull that was 4 inchs wide and 5 foot deep.

Finally I am just talking about hull speed. The dynamics of the hull are just as important if not more. For example, a boat might have great speed but it really hard to tack or it pitchpoles or it hobby horses and stops every time it hits a wave more than 6 inches high , etc, etc etc