It's not only that I cannot see the difference between got and expected, but I have also tried copying the strings between the single quotes into files and then diff'ing these files: no difference.

Even so, it's likely that there are differences in whitespace or line endings that are invisible to you, and may either be ignored by diff(1) or be munged by a copy/paste. cmp the strings yourself and use diag to report the difference. If they compare as different, you have a likely suspect.