Understanding Galera Status wsrep_local_recv_queue_avg
According to the MariaDB KB: Galera Cluster Status Variables:


  Description: Average length of the receive queue since the most recent
  status query. If this value is noticeably larger than zero, the node
  is likely to be overloaded, and cannot apply the writesets as quickly
  as they arrive, resulting in replication throttling.


So it not the average receive queue size for the node's uptime, but since the last time you queried this status.