During my backups I'm finding that rsync is copying all files, instead of just what's changed.

I'm rsyncing between 2 USB external hard drives. One hard drive is FAT32 and one is NTFS. I've examined some of the files and believe that the difference is that there's a 1-second modtime difference developing in some of the files somehow.

Here's an example. These duplicity files were synced from /media/BACKUPHD (the NTFS drive) to /media/VIDEOHD (the FAT32 drive) only a few hours ago this morning. They have not been touched or changed since then, but that 1-second difference in their time stamps has appeared:
Code:
tim@localhost:~> stat /media/BACKUPHD/backups/duplicity/duplicity-full.20110107T145955Z.vol10.difftar.gpg                                                                                        
  File: `/media/BACKUPHD/backups/duplicity/duplicity-full.20110107T145955Z.vol10.difftar.gpg'                                                                                                 
  Size: 681614072       Blocks: 1331280    IO Block: 4096   regular file                                                                                                                      
Device: 831h/2097d      Inode: 2902        Links: 1                                                                                                                                           
Access: (0777/-rwxrwxrwx)  Uid: (  500/     tim)   Gid: (    0/    root)                                                                                                                      
Access: 2011-01-09 12:38:04.478730000 +0100                                                                                                                                                   
Modify: 2011-01-07 16:28:39.000000000 +0100                                                                                                                                                   
Change: 2011-01-08 21:27:10.370064000 +0100      
                                                                                                                                             
tim@localhost:~> stat /media/VIDEOHD/backups/duplicity/duplicity-full.20110107T145955Z.vol10.difftar.gpg                                                                                         
  File: `/media/VIDEOHD/backups/duplicity/duplicity-full.20110107T145955Z.vol10.difftar.gpg'                                                                                                  
  Size: 681614072       Blocks: 1331296    IO Block: 16384  regular file                                                                                                                      
Device: 821h/2081d      Inode: 7398        Links: 1                                                                                                                                           
Access: (0755/-rwxr-xr-x)  Uid: (  500/     tim)   Gid: (    0/    root)                                                                                                                      
Access: 2011-01-11 13:20:14.000000000 +0100                                                                                                                                                   
Modify: 2011-01-07 16:28:38.000000000 +0100
Change: 2011-01-11 07:31:46.000000000 +0100
And here is a file from the same directory that for whatever reason (and it seems to be random) there's no 1-second timestamp difference:
Code:
tim@localhost:~> stat /media/BACKUPHD/backups/duplicity/duplicity-full.20110107T145955Z.vol11.difftar.gpg
  File: `/media/BACKUPHD/backups/duplicity/duplicity-full.20110107T145955Z.vol11.difftar.gpg'
  Size: 681614777       Blocks: 1331280    IO Block: 4096   regular file
Device: 831h/2097d      Inode: 2903        Links: 1
Access: (0777/-rwxrwxrwx)  Uid: (  500/     tim)   Gid: (    0/    root)
Access: 2011-01-11 07:31:45.667345000 +0100
Modify: 2011-01-07 16:31:24.000000000 +0100
Change: 2011-01-08 21:27:42.865310000 +0100


tim@localhost:~> stat /media/VIDEOHD/backups/duplicity/duplicity-full.20110107T145955Z.vol11.difftar.gpg
  File: `/media/VIDEOHD/backups/duplicity/duplicity-full.20110107T145955Z.vol11.difftar.gpg'
  Size: 681614777       Blocks: 1331296    IO Block: 16384  regular file
Device: 821h/2081d      Inode: 7399        Links: 1
Access: (0755/-rwxr-xr-x)  Uid: (  500/     tim)   Gid: (    0/    root)
Access: 2011-01-11 00:00:00.000000000 +0100
Modify: 2011-01-07 16:31:24.000000000 +0100
Change: 2011-01-11 07:32:24.000000000 +0100
Is there some kind of timestamp rounding that FAT32 does that the NTFS driver doesn't do (or vice-versa)?