That volume comes from all the orders prior to 3.5 on the offer... a match price doesnt ignore all previous orders... it equates a fair price (3.5 in this case for example) meaning all those sells at 3.1,3.3 etc will be MATCHED at 3.5... same as bid orders above 3.5 (i.e 3.6,3.7) they too will be matched at 3.5...