The main reason for this is that technology failed.

It is likely that something went wrong on the survey tool side and the status of the survey was not sent back to us.
There was an honesty validation question at the end of the survey.

Either way, when the project is finished we always compare our status with the clients list of completed id's. This allows us to manually update those that completed the survey correctly - we will notify any members when any survey statuses have been updated to completed.