Rsync always do a copy from scratch with a mounted NAS

[RobinFrcd]

Hi,
I'm running this rsync command to back up a folder from my Mac to a mounted folder of my Synology NAS.

rsync --no-compress -avAXEWSlHh --info=progress2 --delete-after /Volumes/SSD1TB/photos /Volumes/NAS/ssd-backup

The thing is, every time I run this command, even if I didn't touch a thing in the folder, the copy takes the same time as the first copy I did:

sent 8.38G bytes  received 113.74K bytes  3.41M bytes/sec
total size is 8.37G  speedup is 1.00

No matter how many times I run the same command, on the same folder, the time is the same. Seems like in that case rsync is not useful at all, but I don't understand why.

Thanks 🙏