Custom Record Numbering Does Not Adjust Even When Existing Records Are Deleted

This behavior has been confirmed as the intended design for custom record auto-numbering.

When Numbering is enabled for a custom record type and no Prefix, Suffix or Minimum Digits is set, NetSuite uses the auto-generated number of a custom record entry as its Internal ID. Since NetSuite uses the Internal ID field as its primary key for all data, the method of using an entirely new number for newly-created records instead of recycling a used number ensures that no duplicate could arise.

If it is really necessary to reset the auto-generated numbers for a custom record type, below are some alternate solutions:
1. Start over with a new custom record type (when the records involved are at a minimum).
2. Disable the default numbering on the custom record type and create a custom field that increments its value upon saving new records.