Why does nova fail create instance with 'ram exceeded' error ?

Issue

• We creating a stack for a project/tenant. This tenant has 512GB of ram quota. The creation fails at 320GB.

Error message from - /var/log/heat/heat-engine.log :

2016-03-18 11:58:41.161 44439 TRACE heat.engine.resource Forbidden: Quota exceeded for ram: Requested 28672, but already used 315392 of 327680 ram (HTTP 403) (Request-ID: req-03cbfd3f-f302-47fb-8ea5-7495a6571d0c)