Why is LFTP "Removing old file" in mirror?

I'm trying to mirror a website (of which I'm the webmaster) using LFTP, for backup. I use the LFTP command (actual paths, usernames, and passwords redacted for privacy): mirror -e -p --verbose=1 /public_html /mnt/lobster3raid//public_html It's working except that LFTP is removing each file from the mirror and then re-transferring it, even if it hasn't changed on the webserver (I get this on every single file every time): Removing old file `/IMG_2003.jpg' Transferring file `/IMG_2003.jpg' Removing old file `/IMG_2004.jpg' Transferring file `/IMG_2004.jpg' Removing old file `/IMG_2005.jpg' Transferring file `/IMG_2005.jpg' **How can I run a LFTP mirror that only transfers changed/newer files?** More background: The destination mirror is on a Windows share at \\\LOBSTER3\raid. I'm doing this on Windows. I had it working OK with Cygwin's LFTP for a while, but lately have been getting permission problems from Windows (apparently Cygwin's LFTP is setting permissions Windows doesn't like). So I'm now using the LFTP in wsl (Ubuntu). I run this batch file from Windows: wsl -u root -- mkdir /mnt/lobster3raid wsl -u root -- mount -t drvfs \\\\lobster3\\raid /mnt/lobster3raid wsl -- lftp -u , -e 'set ssl:check-hostname no; mirror -e -p --verbose=1 /public_html /mnt/lobster3raid/mirrors/1day/public_html'