why does this rsync commands copy again the same files?

[M. Fioretti]

Greetings,

I am running Fedora 16 x86_64. When I connect an external USB drive
and rsync to it my pictures with this command:

rsync -rvt --delete --no-o --no-g /home/z/photo/ /media/Verbatim/photo

because I don't care about ownership and permissions, I just want to
have the same files,with the same names and modification times, in the
same hierarchy of folders.

However, rsync keeps copying certain folders and pictures, and always
the same ones, every time, even if they are not changed at all. If I
rsync, unmount and unplug the drive, then plug it in again and re-run
the command above, rsync copies again hundreds of pictures to the
drive. What am I missing here?

FIY, the source folder is ext4, the destination vfat (see their mount
entries below)

TIA,
Marco

/dev/sda7 on /home type ext4 (rw,relatime,seclabel,data=ordered)
/dev/sdb1 on /media/Verbatim type vfat (rw,nosuid,nodev,relatime,uid=1000,gid=1000,fmask=0022,dmask=0077,codepage=cp437,i