Well, The Grand Exchange updates once a day at 00:00 jagex time, which is... like 13 hours from this post (?). Every items price doesent update ofcours, its depending on the market of the item, how it sells and so on. For example, the Saradomin Godsword, it is kinda hard to sell because no one wants if because all know it is droping in price. Now, as you may see on the grand exchange page on rs homesite, if you look at the 30 day chart, you see that it drops every 6 day, and since it has gone 5 days not and next update is in 13 hours or so, I suggest all to sell it before then if you ask me. So, in summary. All items may not drop in price every day it updates. It is depending on how it sells/buys in the market. Well, I might be very wrong, but this is how I see it.