The curves do look odd, for reasons already mentioned above. The shocks probably have some kind of "velocity dependent" (marketing speak) damping feature where they blow off above a certain speed to help ride quality. With that C/R ratio, you'd probably need it.
I've also seen odd C/R ratios...
2nd what Greg said, mainly because engineers from those types of suppliers often play an active role in developing the vehicle with the OEM's. Spring, bar, and bushing suppliers generally manufacture their parts to the OE spec, and that's it.
I started fresh out of school as a design/release...
I'd be skeptical of anyone claiming to feel a subjective difference between a 4lb spring and 5lb spring, all else being equal.
That said, all else is rarely equal. The actual rate can vary significantly from what is specified. Even the linearity can vary within the stroke of a single "linear"...
