Why, with allegedly formed debt for the Internet - some sites open, but some do not?

Question

Computer networks

Turning on the PC found that the connection status is "no access to the Internet."
Opened firefox, there is a notification - you must log in to this network before you can access the internet and a button open network login page. When you click, you go to a site informing about debts for paying for the Internet. I always cry at the end of the month.
But here's what I don't understand.
Some sites are open. When opening others (from bookmarks, or entering into the address bar), the provider's website opens with information about the debt. If you refresh the page of a site that has not opened, it will not open. But! If you enter its name in a search engine and go from there, it will open.
At the same time, sites that do not open in firefox open in chrome, although there are some intersections.
I would like to understand the principle by which it works. I don't see much logic and it seems rather strange to me.

Answer 1

Some sites may be whitelisted - internal provider sites, bank sites through which you can make payments. Maybe something else is just open at the provider, but something else is cached.
Again, the provider may have a crooked blocking, do not look for special logic, and you should not count on the stability that some sites work even when blocked.
Again, "no internet access" is defined, for example, by access to www.msftncsi.com/ncsi.txt , and the resolution of the dns.msftncsi.com domain. If you block them, then the rest of the Internet can work fine, and you will have an inscription that no internet access. In different versions of Windows, this detection may vary slightly.