Grand exchange updates

Can someone please explain why merchants like to buy their items right after grand exchange updates its prices.

I haven't played the game in a long time so I pretty much forgot how GE works. ^_^

 

If items are rising in price, and there is enough demand for it, right after the G.E. updates people will be selling for the old price instead of the new. This results is in a big profit if enough items were bought.

 

It's because of the new price you can get items at the old price that people listed them at before the g.e update!