It is.
[spoiler=My Synopsis]
I have not played BBS (I wish I had, then I'd know more. CURSE MY NOT HAVING A PSP!)
Ahem.
As I see it, the first KH was simple. Kingdom Hearts was a door with great power, and Ansem wanted it. He used Riku to do so. Sora was the keyblade wielder so he had to stop the Heartless. He defeated Ansem and stopped him from devouring the Hearts of all worlds. Simple. The Heart is now freed.
KH:CoM added in the Organization. And also showed that Riku was also a keyblade wielder (I think), and his fight with darkness.
KHII added Nobodies, which are the captive shells of those lost, twisted by magic unless the one who lost the shell was powerful enough. Sora comes back, and defeats Xemnas, Xehanort's Nobody (much like Ansem was Xehanort's Heartless). Good, the body is now free. The freed body and heart can recombine.
Go back to BBS, from what I understand, and it fixes a lot of confusion. Such as how the hell the whole deal started. With Xehanort (actual Xehanort) appearing, and showing the genesis of the Organization (I think), and explaining things regarding Sora and Roxas (with Vanitas and Ventus). I could be wrong on this, as I haven't played it. But this is how I understand things.
That make sense?
Also, 358/2 Days was just a way to explain various events regarding Roxas to clear up some confusion. I still haven't beat that (save was erased at Day 250 or something) so I'm not sure about Xion, aside from what I've read in spoilers.
[/spoiler]