The lower hull is modeled as a primary surface with a square stern, and then the rear quarters (corners) are trimmed with other surfaces. This significantly reduces the complexity of the primary surface compared to trying to incorporate the curved quarters directly into the surface, and also eliminates singularities and degeneracies which can cause problems.
The green surface is the primary surface of the lower hull. Curvature continuity (G2) was apparently sufficient which is common for most boats. Given the final fairing of the physical hull is done manually any higher degree of continuity is not needed for the physical build. Also a lot of “Class A” surfacing on the internet only uses G2 continuity between individual surfaces. Using degree 3 multi-span surfaces provides automatic G2 continuity between spans (except where control points are stacked.) Individual “Bezier” spans could be used with G2 continuity between surfaces but addition control points would be needed.
Degree 3 provides more local control of the surface shape than higher degrees. Sometimes more local control is undesirable because it is easier to introduce undesirable local deviations from fair. Sometimes more local control from lower degree is desirable because a given shape may be achievable with fewer control points. For example the tight radius near the stern of the green surface would require 8 control points if the surface was degree 5 in that direction rather then the 6 control points needed by degree 3. The designer/modeler apparently decided degree 3 with 6 control points in each direction was sufficient to achieve the desired shape.
The sky blue surface may have 6 control points in the U direction because that number of control points was needed to achieve the desired shape. Or that number may have arisen form a particular workflow or tool used. Based only on the illustrations it looks to me like 4 control points may have been sufficient.
I don’t have a good idea of reasons for the structure of the yellow and red surfaces or even why there is both the yellow and red surfaces rather than a single surface.