Good catch. I see this issue was mentioned but not much discussed when the R came out.
I just checked both the 5DIV and the R. In AV Mode, on the 5DIV, my VF shows +/-3, but allows me to keep compensating beyond those numbers another full stop, for +/-4. The R just stops compensating when the little gauge's needle hits 3.
But in Exposure Bracketing, my 5DIV shows +/-8 stops, while the R shows +/-6 stops.
As to "why," my only guess would be Canon decides what specs to put in various cameras.
On a personal use note, Saturday I took the R out for dawn landscape photos, and I really enjoyed using it; however, I also brought my 5DIV, using its Live View. I don't see any edge going to the R for landscape, but I like the perception of better weather sealing on the 5DIV, and the chance to use it for quick moving wild life if the opportunity arises. Still, I do think the R is just about the best PORTAIT camera I've used, as long as the subject is not moving too erratically...