Well... looking at the amount of free space on your harddisk, and, looking at the size of the files to be defragment, it seems it simple has not the space to create those unfragmented files. The 'gap' of free space between the other (already defragmented files) is just small to fit in a new file of the size of , for example, 394873KB.
Not sure, but I think I am close to the truth