I guess we could assume it's a different Gravus - the game mentions there being many Darth Jugoran's. Unless the book specified that this is the Gravus that destroyed Taris, in which case it's just another example of DK butchering a character to help his own "pet" characters. (coughMeetracough)
it did, when the Dark Council was talking about putting him on it they mentioned what he had done on Taris. They turned this one into a pyscho who lost his cool and was more like Thana and not like the calm headed, Gravus on Taris that nothing ruffled his feathers.