Question about cause of certain RP symptoms

Wondering if anyone actually knows (that is, not just guessing) why RP causes decreased contrast in one's remaining central vision. I asked my retina doc and got an obviously BS answer that cones can't distinguish contrast as well as rods. I am not a fan of doctors who don't have the phrase "I don't know" in their vocabulary. I also googled and came up with a zip. FYI, I'm asking only because I'm curious. With me, the lack of contrast has existed since probably around the time I was diagnosed, it has progressively gotten worse as my vision has deteriorated. In the distant past I was a biologist, and I just can't figure out why retinal changes would cause this. Seems more like something that would be caused by problems with the lens or cornea