No diff found

I do not understand why but when I launch a sync no differences are found.
But If I do a winmerge the right one is empty as on the picture.

Here the second picture

You are synchronizing local directory to remote one. As your remote directory has a newer version of the APC.php, there's nothing to synchronize.
Either you want to reverse the direction. Or if you want even older files from the local/source directory overwrite the newer remote/destination files, select the "Mirror files" mode.
https://winscp.net/eng/docs/task_synchronize_full