In the API documentation for the 3.0+ MongoClient options:
http://api.mongodb.org/java/3.0/?com/mongodb/MongoClientOptions.html

When using a List of servers it states:

If this is a list of mongos servers, it will pick the closest (lowest ping time) one to send all requests to, and automatically fail over to the next server if the closest is down.

But with 3.0+ this is no longer true, as described in
MongoClientOptions.getLocalThreshold():

Gets the local threshold. When choosing among multiple MongoDB servers to send a request, the MongoClient will only send that request to a server whose ping time is less than or equal to the server with the fastest ping time plus the local threshold.