Hello Knowledgable,
I am assuming on of those screenshots is the cluster resources and other on the host with the vCenter running on it (which accounts for ~10GB usage).
The other 13GB is being consumed by the overhead from the Disk-Groups - how much this requires is based on whether is it All-Flash or Hybrid, size of cache, number of capacity-drives per Disk-Group and then multiply this by the number of Disk-Groups:
BaseConsumption +
(NumDiskGroups * (DiskGroupBaseConsumption + (SSDMemOverheadPerGB * SSDSize))) +
(NumCapacityDisks * CapacityDiskBaseConsumption)
VMware Knowledge Base
The amount of Memory overhead these require is mandatory and cannot be avoided or reduced (without changing any of the set-up).
You have very small cache-tier devices and low cache:capacity ratio here, I do hope the caches are good performing models and/or you aren't expecting the workload on this cluster to be too taxing.
Bob