Account for when the lhs of `difference_count` has a greater length than the lhs

[SkiFire13]

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zip truncates the length of the iterator to the shortest of the two, but this is incorrect for difference_count since that might ignore some bits in the tail of the lhs that should be present in the difference. The solution is to include a sum of those. The test for difference was also extended with a case that would trigger this issue.