My guess would be around your note. If someone mistakenly has two SSNs (due to fraud, error, or name changes), combining DOB helps detect inconsistencies.

Some other possibilities, and I’m just throwing out ideas at this point:

• Adding DOB could help with manual lookups and verification.
• Using SSN + DOB ensures a standard key format across agencies, making it easier to link records.
• Prevents accidental duplication if an SSN is mistyped.
• Maybe the databases were optimized for fixed-length fields, and combining SSN + DOB fit within memory constraints.
• It was easier to locate records with a “human-readable” key. Where as something like a UUID is harder for humans to read or sift through.