Short answer
Passenger capacity changes when accommodation changes. Refits, class reconfiguration, immigration patterns, regulations, and cruising use can all alter the number of berths without changing the basic hull.
Why capacities differ
Class reconfigurationCabins could be reassigned between first, second, tourist, cabin, or third class as markets changed.
RefitsPublic rooms could become cabins, cabins could be enlarged, and old steerage spaces could be rebuilt for fewer but more comfortable passengers.
RegulationSafety, sanitation, and accommodation rules could limit how densely spaces were occupied.
CruisingA ship marketed for cruises might operate at a lower practical passenger count than its maximum scheduled-service capacity.
Maximum capacity is not the same as normal load
A published capacity may represent the number of berths installed, an officially certified maximum, a publicity figure, or a particular class plan. It does not tell us how many passengers usually sailed on a voyage.
Class totals matter
A statement such as “2,000 passengers” can conceal a major change if the composition shifts from large third-class dormitories toward lower-density tourist accommodation.
Best practice: attach capacity figures to a date or configuration whenever a ship underwent significant accommodation changes.