When you create a pod with a memory limit, Kubernetes looks for a node with the right labels and selectors that has enough of the resource types, CPU, and memory, that the pod requires. Kubernetes is in charge of ensuring that the total memory request of the pods on a node is not less than the pod's total resources. This can sometimes result in unexpected outcomes, as you can have node limitations reached in terms of capacity, even if the net utilization of a pod is low. This is a design of the system in order to accommodate varying load levels.

You can look through pod logs to find out when this has occurred:

$ kubectl describe pod web| grep -C 3 Events
Events:
FirstSeen LastSeen Count From Subobject PathReason Message
74s 15s 2 {scheduler } FailedScheduling Failed for reason PodExceedsFreeCPU and possibly others

You can address these issues by removing unneeded pods, ensuring that your pod isn't larger as a whole than any one available node, or simply add more resources to the cluster.