It should be noted that that fight was the first time Grievous ever dished out the spinning four-arms against Kenobi, as far as I'm aware. Obi-Wan wasn't well equipped to deal with that in an environment that was absurdly narrow and small, which favors Grievous greatly. Overall though, Grievous presenting a challenge to Kenobi doesn't mean he's on-level with Kenobi. Kenobi in TPM, as a 7, pressed Maul, an 8, and Anakin in AotC /TCW movie, as a 7, pressed Dooku, an 8. To me, the best indicator to see where you're at is if Grievous can do likewise (so, if we want to assume incorrectly that he's a 7, see how he does against 8s), and given he's losing to Ventress (I don't think nexus' in Canon work like they do in Legends, and even if so, there's no indicator or implication whatsoever that the nexus was the deciding factor there), I really doubt he's coming close to pressuring Dooku in any form. Thus, I associate Grievous' performance against Kenobi similar to Kenobi's against Maul's: Grievous, a 6, is able to compete with a 7, although he's inferior, thus why he generally has to resort to trickery and/or aid to help him.