I noticed that when reservations happen when using the Capacity Scheduler, the UI and web services report the wrong total memory.
For example. I have a 300GB of total memory in my cluster. I allocate 50 and I reserve 10. The cluster metrics for total memory get reported as 290GB.
This was broken by https://issues.apache.org/jira/browse/YARN-656 so perhaps there is a difference between fair scheduler and capacity scheduler.