I think its because usually in the scans when these types of games are announced, even if they don't specifically state or even show any new content, there will at least be a blurb along the lines of "additional content from international english release", or something of that sort, there's not even a mention of that here.
I've heard there was a Nomura interview that went along with the announcement in Famtsu- any news on that?
So in terms of the feasibility of adding new content (UMD space aside), let's take a look at KH1's timetable.
Released in Japan in March, international in September and Final Mix in December.
Really, that's not much different than BbS.
January in Japan, September international, and Final Mix some time in the winter.
There's still a three (or more) month span between the English release and the Final Mix, giving them the same amount of time they had for KH1FM. They very well could add more content.
And it wouldn't surprise me if they did (not saying I'm necessarily expecting it).
My guess is that they could have a planned obsolescence going- that they've been working on additions in general since the original release in January, but had chosen to only put so many in the English release. And then they put the rest into the Final Mix.
That is, they intentionally hold content back to make each release inferior to the next.
If they plan on adding even more content to Final Mix, there's not a doubt in my mind that they have been working on it prior to September.