You are seeing a difference in the behavior of Linux's multi-thread implementation. It changed sometime around the RedHat 9 to Fedora Core days.
Most likely you have an older version of Linux that you run 3.13 on, and have a newer version of Linux that you run 4.5 on.
Read the man page on the "ps" command and look for info about threads.
You can run "ps -eLf" to see information about each thread, for instance. Running "ps -eLf | grep nsd" will show you that multiple threads are in fact running.
This article from IBM DeveloperWorks will have more information: http://www-128.ibm.com/developerworks/linux/library/l-threading.html?ca=dgr-lnxw07LinuxThreadsAndNPTL
