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To make it concrete: consider a cluster of ten machines with 256G of ram. Team A and team B both schedule jobs with a requirement of 129G of RAM and five replicas each. The replicas get scheduled, one on each machine. Team C wants to schedule a task that takes an entire 256GB of ram. If they don't have sufficient priority to preempt the other jobs, then they will not schedule.

In real heterogenous-workload production clusters, every available machine likely has several VMs scheduled on it if the cluster isn't idle. There is never a full machine that's free unless some special effort has been taken to make it so.