Short answer
Casualty totals differ because the underlying headcount was sometimes uncertain before the disaster even occurred, and identification continued afterward.
Why totals change
Incomplete manifestsLate bookings, cancellations, transfers, infants, servants, crew substitutions, and undocumented travelers can complicate the starting count.
Passenger versus crew totalsSome sources combine everyone aboard; others report passengers and crew separately or omit nonstandard personnel.
Missing versus deadEarly reports may classify people as missing until survivors are identified or bodies recovered.
Contemporary reportingNewspapers often published provisional numbers while rescue and reconciliation were still underway.
Name duplication and spellingTranscription, aliases, married names, language differences, and duplicate records can affect later reconstructions.
Later archival researchResearchers may reconcile manifests, inquiries, burial records, immigration records, and family documentation decades later.
Which number should a reference site use?
The strongest total is the one supported by the clearest documentary reconstruction, not necessarily the most frequently repeated number. Where major reputable sources still disagree, a range or explicit note of disagreement can be more responsible than choosing one figure without explanation.
Human-history standard: casualty figures represent people, not trivia. A discrepancy should be handled as an evidentiary problem rather than rounded away for convenience.
Primary records matter
Official inquiries, passenger and crew lists, rescue-vessel records, consular documentation, burial records, and archival correspondence can each answer different parts of the question. No single record type is automatically complete.