Steam market order placement question

Hi I have question about this. So how do buy placement works?

like I saw this guy put the buying order with the price of 2.31 and when I saw that I put the buy offer of 2.5 something like that. And I kept checking my buy offer still higher than him for hours and then somehow his buy offer is gone and the chart shows this.

How is this even possible and how does it works? Shouldn't the higher price to be picked up and purchase first?