The cluster is running SLURM's scheduler with:
The “Fair Share” algorithm assigns a priority to submitted jobs which is inversely dependent on the amount of cluster CPU time consumed by the user submitting the jobs in recent days. These priorities apply only to jobs waiting in the queue, they do not affect jobs which are already running.
The scheduler does not check which nodes are busy and try to avoid them. This has an advantage in that it tends to leave some nodes empty for people who need a whole node.