Fg isn’t elastic, it has a set dollar value exchange rate because you can buy it for the same dollar value any time. Actually what this leads to is a massive opportunity to arbitrage currencies across games
The value of FG is elastic relative to in game currencies though. Sure you can buy it directly from the website at the set rates, but that doesn't prevent it from being impacted by games and their updates.
OSRS on this site isn't valued like OSRS on other sites where you can purchase it. People are looking for quick sales to put the fg to use in other games, people are looking to cash out from their BnP adventures, etc.
Further to that, the value of OSRS, long term, will only continue downwards as more gold is injected into the game.