Exchange 2013 Logs Not Flushed

BE 2010 R3 on server 2008. We have the exchange license installed and backing up our new exchange server. However, when i look at the exchange server log files, they are not flushed/removed. My understanding is, BE will committ the logs and delete those that are committed. Is this correct?

The full backup is using full DB and logs backup (flush logs) and the incremental back is using incremental logs backup (flush logs). But the 36GB of log files remain. What are we missing?