The InnoDB buffer pool caches queries; if you have less usage on one of the nodes then you're going to have less cache used.
If you're not properly load-balancing queries then the amount of RAMcache in use on each node would be different. Basically, it's normal to see nodes with different amounts of RAM consumption, they will never be the same unlessIf you are running the exact same queriesjust turned on alla node and added it to the nodes atcluster the exact same time.
The InnoDB buffer pool caches queries; meaning if you have less usage on oneamount of cache would be different than the nodes then you're going toothers one that have less RAM usedbeen long running.
It doesn't matter if you use Galera in this case as this is a specific InnoDB question; sure you could be using Galera, but it has nothing to do with this caching question you've asked.