Message from website page

I have a list of websites I visit regularly, and whose "safety" I  check with Sucuri SiteCheck very often. 

One of them is "nationalgeographic.com", and also today before visiting it I rechecked it with SSC and it resulted safe, as usual.

When I clicked on the link in my list, the following message appeared on the website page (translation of the first line is in red):

 

11.21.13 Message from NG website page.JPG 

 

Please note that I clicked on "nationalgeographic,com" and the URL of the page is exactly the same as you can see, while the message says: the page at the address "players.nationalgeographic.com".

Why that  players which was not part of the link in my list?

 

I closed the message with Alt+F4, clicked again on the link in my list, and the page opened without the message. I tried again, and again no message.

I then closed and reopened the browser (Chrome), and tried again: again no message.

 

I've never seen this message before. Can anyone please explain to me what it means (anything to worry about?), and why that difference in the address? 

 

Thanks,

Laura