Why is the Buy_Limit allways use the highest price?

Buy_Marked is disbaled...

I want buy by the API

bid + x% = Y
Y = MAXMIMUM

But i buy with the highest price, not filled up till the highest price is reached...

the uper was buyd by hand, the lower with api
http://imgur.com/aVcqev2

Poloniex works fine... they try to buy with lowest price, and fill up, till maxprice is reached...