NTFS bitmap table not continues

Hi, we have a NTFS filter driver which works fine on most NTFS systems,
but on some systems we found out it is not reading the disk free space
corretly. Our research indicates the bitmap table is not “continuous” like
most the other NTFS systems we have seen. What could have caused this
issue with the bitmap table? But the XP system seems to work ok other than
a little bit slower. Thank you very much in advance.