This sounds like a classic case of a shared IP system. Basically when someone enters www.yourdomain.com
or yourdomain.com in their web browser, a DNS server is queried to find the IP address of the site. So for eg.
HostInvestigator.com - 184.108.40.206 www.HostInvestigator.com
The request is then direct to server with the corresponding IP address. Some hosts offer a unique IP with every domain, so whatever domains point to that IP the webserver will respond (with the correct site). However due the shortage of IPs many web hosting companies use shared IP whereby more than one customer shared the same IP.
- 220.127.116.11 www.whatdayisittoday.com
- 18.104.22.168 www.mycompany10.com
- - 22.214.171.124
Then when the web server receives a request, it looks for the header inside the request to see what site sould be sent. In this case both www and non-www point to the correct webserver, however because they are using a shared IP system the webserver also has to be setup the respond to the correct domains. Currently your webserver will be setup like:
Ask your web hosting provider to modify their web server configuration so the non-www version of your domains points to the correct directory/site. Or better yet ask them to put you on a static IP.
I'm sure their are some better explanation on the net, sorry for the confusion