Extra NTP server address

I run my own NTP server, which can be reached with both IPv4 and IPv6.  However, when I check with Wireshark, I see NTP requests going not only to my server, but also to an IPv4 address on the Internet.  This happens with both my desktop system and ThinkPad, but they have different external addresses, in addition to my server.  I use the host name for the server address in NTP Configuration. Any idea where that other address comes from?