rsync re-syncs unmodified files (was: A question about rsync's behaviour)

[Rodolfo Medina <rodolfo.medina@gmail.com>]

When using rsync from pc to usb stick, it re-syncs unmodified files, which is
annoying with large backups.

In rsync man page, I found the following:

   When  transferring  to  FAT  filesystems  rsync  may re-sync unmodified
   files.  See the comments on the --modify-window option.

And, the comment on the --modify-window option:

--modify-window
              When  comparing  two  timestamps, rsync treats the timestamps as
              being equal if they differ by no  more  than  the  modify-window
              value.   This  is  normally  0 (for an exact match), but you may
              find it useful to set this to a larger value in some situations.
              In  particular,  when  transferring to or from an MS Windows FAT
              filesystem (which represents times with a 2-second  resolution),
              --modify-window=1 is useful (allowing times to differ by up to 1
              second).

In fact, adding the option `--modify-window=2' seems to solve the problem.
However I wish to better understand the problem and to understand what I'm
doing.  When listing files with `ls --full-time', no difference at all is shown
between pc and pendrive.  So, why and what does `--modify-window' work for?

Thanks for any help
Rodolfo