Why does "chronyc sources" output unexpected NTP servers

Issue

chronyc sources outputs NTP server which is not configured in /etc/chrony.conf.

# cat /etc/chrony.conf | grep ^server
server clock01.util.phx2.redhat.com iburst 
server clock02.util.phx2.redhat.com iburst

# chronyc sources
210 Number of sources = 3
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^* clock01.util.phx2.redhat>     1   6    17    61    +47us[  +54us] +/-  125ms
^+ clock02.util.phx2.redhat>     1   6    17    62   +133us[ +140us] +/-  125ms
^+ ns01.rdu2.utst.redhat.com     3   6    17    61  -5084us[-5084us] +/-  157ms  <=====