what is this?
why is there a difference?

The mouse-over bar indicates the cumulative amount of BTC (and the coin you want to buy) from all order till the one selected.
yeah i know that. but you see the difference between mouse over and buy? mouse 14.9 and buy(see above) 15.2 . that should be the same?!
The buy box uses the highest price to calculate the total price, so the coins offered for 5 satoshi are also counted as 6 satoshi, but in the end you should only pay 5 satoshi for the coins offered for that price... so the price in the bar should be the right one then.