There are 2 possible reasons for this issue:
1. The account is in an "outstanding" state. The sync is responsible for this. If the account doesn't exist in Active Directory (AD) the sync sets it to "outstanding".
2. If the account DOES exist in AD the account may be marked for deletion. You can confirm in Manager in the result list of accounts - it will be marked with a red cross if marked for deletion. In this case there should be a process “…Delete_Physical” in the JobQueue which will execute the deletion after the configured delay.