Why Do Some Records Form Single Clusters Despite Sharing Field Values with Others?

one more question: I have some examples where I would have expected a record to match to other records but it ends up in its own one-record cluster. These examples share a value for at least one field with another record (or records) (e.g. first_name is shared with 3 other records, but last_name is different). These values aren't super common, so I doubt that it's the ignore the most common terms strategy that's causing this. Is there anything else obvious that I should look out for that could cause this?