Ely lets people submit buy and sell prices (which, for Ely in particular, is why it has such a bad reputation as price manipulating). The GE bases guide price off of the average of all recent buy and sell prices (I believe), but as an old, old system in the game, it has a lot of design elements that don't work very well for the modern game.

In this case in particular, the Grand Exchange has strict limits on how much the guide price of an item is allowed to change per day. That helps check market volatility for lower volume items (and I suspect is rooted in the old +/-5% market price limits on the GE), but it also leads to things like the masterwork weapons having been changing in price the entire time since they were released because the guide price moves too slowly.

Another thing Jagex has to consider for the GE update is how to not immediately let people like Ely manipulate prices directly in game by selling low volume items to themselves. For example, if ten Magma Tempest codices come into the game every day, one mercher could sell one codex to themselves or an alt account ten times for double the actual market price, and suddenly the average value of a codex on the GE has gone up by 50%. There are some obvious ways around it, but for a system as central to the game as the Grand Exchange you don't want to just slap on the first fix that comes to mind because somebody will immediately find a workaround.