Okay, so I have a theory to tie in 2003 TCW and cannon TCW/ RotS Grievous.
I suppose someone could put this into more detail if they wish. But how about after being wounded so ...grievously... (I'll show myself out) and being mostly replaced with cyber genetic parts, he was hindered too much to fight. Think Vader immediately after being put in the suit, but instead of just mechanical limbs and a respiratory system, it's every single part of him except for his organs and eyes; so it would be much harder for Grievous to adjust.
He trains and relearns the basics of combat, and gets pretty good to the point to where he can take on and beat one Jedi easily as long as they weren't too powerful like a council member or something. To help even the playing field though, Sidious and Tyrannus get together once every few months (to ensure security by not risking discovery) and use Sith alchemy to create some serum or potion that temporarily heightens Grievous' senses and makes him super calm yet deadly in the middle of battle, unlike how he would normally fight. Also I imagine that it would be accompanied by a temporary suit power upgrade that speeds up his limbs.
Sidious and Tyrannus can only make small doses at a time, but once they have enough, Grievous can take on more than enough Jedi at a time and win easily. Notable times he uses this being the first time the CIS introduce this new Jedi slaughtering weapon, and the Battle of Corusant where he has the pivotal role of capturing the Senate. With the death of Tyrannus and the war closing to am end, Sidious no longer has the means to make more of the serum using alchemy since he's by himself, nor does he wish to as the war is ending the way he plans it to. Therefore Grievous remains weakened, especially with his recent wound of Windu's force crush on his chest, making him even weaker when Obi-Wan fights him (also Obi-Wan's a council member and one of the Jedi's best).
So the fight between him and Obi-Wan actually makes sense. And with this theory, so does seeing both versions of Grievous.