Rsync sends again already existing files

[Stephane Ascoet]

Hi, /media/clec1enextsanstampon/gigamopourcdas4/ contains an old copy of files 
of /media/mo. I'm updating with "cd /media/clec1enextsanstampon/ && rsync 
--backup --backup-dir=/media/clec1enextsanstampon/elementssupprimesdepourcdas4/ 
--del -hPrStvXz --progress /media/mo/ /media/clec1enextsanstampon/gigamopourcdas4/"
But unmodified and already in place files in 
/media/clec1enextsanstampon/gigamopourcdas4/ are re-sended from /media/mo 
instead of being skipped, why?

-- 
Sincerely, Stephane Ascoet