It compares 9 nodes of Couchbase Server to 9 nodes of MongoDB. Couchbase Server reads and writes to every node, but MongoDB only reads and writes to primary nodes. That's the problem with active/passive topologies. MongoDB can read from secondary nodes but only if a) you don't require strong consistency or b) you don't mind degrading write performance - it would have to replicate to all secondaries nodes before the write finishes.