I can't speak to Arista II paper specifically, but a couple of issues back, there was a review in Photo Techniques magazine of several different VC papers. Most didn't achieve quite their claimed range of contrasts. As I recall, most seemed to stop increasing as the contrast approached or exceeded the supposed grade 4 but were better at the lower grades; however, I believe one or two showed a pattern similar to what you're reporting. Unfortunately, I don't recall the exact issue or which papers did worst in these tests, and I'm not sure where I put that issue, so I can't look it up quickly.