How do I partition my computer for Boot Camp?
Yep, that's kinda odd.  Might be because it needs to partition a contiguous chunk, for efficiency reasons, for Windows.  If that's the case, and it's got some immoveable files within that contiguous chunk, I could see it having issues.

I wonder if it would be possible to run boot camp assistant from the OS X disk instead.  If you can, those files could probably be shifted as you wouldn't be booted from the 1st partition anymore.

Alternately you could create a bootable partition on an external hard-drive and do the same from a fully functioning OS X installation.  Finally, the last resort would be to use a 3rd party utility to add that NTFS Bootcamp partition.  I don't recommend that route.