1. If you use iCloud to synchronize Safari with a mobile device, you may be opening a bookmark to the mobile version of the site. Another possibility is that you're getting a Handoff activity from the device.
Remove everything from the URL except the domain name and try again. For instance, if the URL is something like this:
http://www.example.com/mobile
or
http://mobile.example.com
change it to this:
http://www.example.com
2. From the Safari menu bar, select
Safari ▹ Preferences... ▹ Privacy ▹ Remove All Website Data...
and confirm. Test.
3. If the above steps don't solve the problem, load CheckMyIp.org and post the "USER AGENT information" shown.