ddrescue on SD-card: Why is the output differing but no errors given?

I'm doing a backup operation on a SD-card. This card has been used for almost 8 y with regular write operation. Chances are that it is damaged somehow. That's why I choose ddrescue to get the content. # sudo ddrescue /dev/mmcblk0 /tmp/backup.img ddrescue runs with no perceivable slowdowns (4 GiB) and gives no errors. However I was curious and ran # sudo cmp /dev/mmcblk0 /tmp/backup.img which ends up in a byte differing amid the file. I thought I could do another image and compare them. However the images keep differing from the card and from each other. How comes that I cannot see any errors, warnings or strange behaviour during the rescue operation? dmesg doesn't show any errors either. What should I assume? That a backup isn't possible? What should I do to get an image with most similarities to the original? **Update** I made eight backups, all differ. There is an extremely large number of differences. If I do a cmp -l | wc -l between any two of the eight backups I made, the number is always greater than 100000. So showing the output doesn't make much sense. The differences start from varying points in the file but there seem to be some statistic correlations. I'm not done analyzing that yet.