Truncating Exchange 2010 Logs

Your settings sound fine - as as you say these are the details from the Admin Guide:

Full Backup

This schedule type backs up the Exchange Server database and associated transaction
logs. Exchange truncates all committed transaction logs after NetBackup notifies it that
the backup succeeded. In replicated environments, the truncation is scheduled and does
not occur immediately.

Differential Backup

Includes the changes since the last full or differential incremental backup. After NetBackup
notifies it that the backup succeeded, Exchange truncates all committed transaction logs.
The truncation of the transaction logs sets the context for the next backup.

So to clarify:

The logs should get truncated but this is scheduled by Exchange so may not happen immediately.

If there are any issues with your DAG and log replications are not fully up to date they they cannot be truncated - check your event logs on the Exchange Server for ESE and other messages at the time of the backup completion to give you an indication of what is happening and any issues being experienced

Hope this helps