Why can abundant records still hide a ship class?

Why does so little usable information survive about some sister ships?

The material may be plentiful but dispersed among separate building archives, making each vessel look like a different class.

Some sister ships are hard to reconstruct because the evidence is fragmented rather than absent. Each builder's archive may preserve a substantial record of its own vessel and manufacturing process. Read in isolation, those collections can make closely related ships appear to belong to separate classes. The research problem is connecting dispersed records into a class-wide account.