File matching operations typically support multiple key fields for comparison. Using multiple keys allows for more sophisticated matching logic where records are compared on several criteria in sequence (primary key, secondary key, etc.). This provides finer control over the matching process.
When comparing files using the MATCHED keyword, the comparison logic supports specifying multiple key fields, not just a single one, so records can be matched on a composite key made up of several fields. Restricting matching to only one key field would be too limiting for real-world file comparisons where records are often uniquely identified by more than one field.