MariaDB Galera setup with drastically different amounts of memory used on cluster nodes - why?
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 cache in use on each node would be different. If you just turned on a node and added it to the cluster the amount of cache would be different than the others one that have been 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.