This KB details information around why the updated field on a record can be different from what is seen in the audit history.
User observes that the updated time on a record is different from what the audit history of the record displays
Users may have noticed in some cases that a records 'Updated' (sys_updated_on) date/time is slightly different (usually a few seconds) than its Audit records 'Update time' (update_time). This is because the 'Updated' field is the date/time of the beginning of the transaction (when the Save/Update button is clicked) and the Audits 'Update time' is when the transaction completes. In most cases transactions complete within the same second so there usually is not a discrepancy. However in some cases, there maybe many business rules that can take an extra couple seconds to complete so a user might see a difference in time between the updated field and what the audit history shows. This behavior is by design.