When would a job with lower priority be granted resources before jobs with higher priority?

Using Netbackup 7.5 with one master/media server, I have an NDMP job with a job priority set to 90 that's being granted tape resources before NDMP jobs I have set to a job priority of 95. Aside from priority, all of our NDMP policies are configured the same: 

- Full Backup

- Same retention level

- Same start window

- Same destination STU 

- Multiplexing level set to 1 

- Multistreaming is disabled

The storage unit is configured with 2 concurrent write jobs with multiplexing enabled and streams per drive set to 6. We have one robot with two drives. Why are lower priority jobs being granted resources before higher priority jobs? 

Thanks in advance for your help. 

Chuck