Hi,
This is expected behaviour. Because the order of the config servers in the --configdb parameter of the mongos matters, and the mongos will always use the first of those config servers when addressing the configDB, there will be more traffic to the first config server in the string. With the balancing switched off, no moveChunk operations will take place and the chance of getting a staleConfigDetected (and therefore needing to reload the config from the configDB) reduced to nearly nothing. This means that the traffic to the first config server will be much reduced.
Please note, you cannot simply randomise the list of config servers per mongos. The --configdb string needs to be the same across all mongos'.
Regards,
André