Well, the image is actually bad, probably the disk itself is bad.
As Steven described, the program checks the first 16 bytes of the weak sector. Reads the sector twice (using xbios call) and computes an 8-bit checksum. If the checksum match the protection doesn't pass.
The problem is that the sector is not weak enough, is too stable. According to the image there is a single weak bit on those 16 bytes. So chances that two readings will be identical (let alone that a simple checksum can match anyway). And this is for the image that works sometimes. Other images don't have a single weak bit on the first 16 bytes, so they always fail.
This indicates a bad duplication. The sector is probably weak enough depending on which drive is read. I have seen this already in a couple of other cases.