And from what I understood they only had concept art for Grievous to go off of, so they had to invent his personality on their own.
But to be fair, I understand why George wanted to make this General character a mustache-twirling villain that runs away and gets beat over and over. It fits into that villain archetype of old serials. I don’t think people would have been as disappointed if Grievous had just been set up that way in the micro-series as well.
Growing up I always rationalized the characterization difference as Grievous being pretty powerful right after his transformation, but slowly became weaker as his remaining organics struggled to exist within his cyborg body.
At the end of the micro series he gets injured by Mace, which presumably causes his cough, and which I interpreted as a kid to inform his more cautious demeanor in ROTS. Honestly I don’t even have that much of a problem with how Grievous is portrayed in ROTS. Just don’t think “charismatic” is a word that applies to him.