Why do duplicate files have different checksums?

Solution 1:

what are clearly duplicated files have different checksums.

If they have different checksums, they are clearly not duplicated.

It is possible that different files have the same checksum (in fact, since there are a finite number of checksums but an infinite number of possible files, there must be an infinite number of different files that have the same checksum). But it is not possible for identical files to have different checksums.

What causes the different checksums?

The cause of the checksums being different is that the files are different, not duplicates.

It seems clear to me that the archives titled "SonyPersonalFolderBackup (*).pst" are copies of the original. Note the byte counts are equal.

The byte counts of a file containing the string hello and a file containing the string bybye are also equal, yet clearly, the two are not identical.

Why is this, and what duplicate removal strategy, excluding name-based, would help me to delete them?

First, you need to define what you mean by "duplicate". Obviously, your definition of "duplicate" is different from the definition of the tool you are using (which probably uses the definition that "duplicate" means "identical"). Since you consider files to be duplicates that are clearly not identical, you need to define what you consider duplicates.

If you don't know what you consider a "duplicate", then you also can't define a command which knows that.