When the NCOA is ran it looks for any addresses that submitted a National Change of Address, if any of the addresses were updated after the date the NCOA was ran in the database, they would not have been updated on your NCOA.
The NCOA cannot catch every address that may no longer be valid. This is because it uses reported Change of Addresses. If someone does not report an address change, the NCOA may not catch it, but the Post Office may still return it as a bad address.
Also, if the Account Name contains multiple prefixes (such as Mr. and Mrs. Smith), the NCOA may not catch reported moves. The NCOA tool can look past single prefixes on an account name (like Mr. Smith). If any of the accounts the NCOA may have missed have multiple prefixes, that could be why they weren't updated.
If you believe the NCOA missed an address that was reported as changed, please contact support and provide the Article ID for reference to your issue.