From how you write, it sounds like you know what you are writing about.
I must admit I do not.
But, forgive me this further ignorant conjecture:
The game has data about the player saved on the server. Things like XP, location, membership status, items in bank, etc. No need to list everything, especially since I probably can't even guess it all. Point being for saying so, that data is not stored on the client side.
The website has access to that data. It has a highscores. There is also Runemetrics, which gives fairly detailed information about account activity. From my brief reading on the Wiki, it seems that Runemetrics is updated about your activity every 5 minutes.
I cannot claim to know the difference between Runemetrics knowing I just gained 50k smithing XP and Runescape.com/m=forum knowing I am currently logged into the game, but it really does not sound that different to me as a layman.
Can you explain why Runemetrics couldn't just keep me logged in if I gained some XP in the last 5 minutes?
11-Jan-2021 06:23:54