What mother board are you using, and which SATA ports are you using?
Your results look like that drive is connected to a third party SATA controller, like Marvell or ASMedia. The majority of those chipsets are connected to one PCIe lane, at 5Gb/s, which is shared between all the ports those chips provide. They do not truly provide the best SATA III performance at all, only the Intel 6 and 7 series chipsets with SATA III ports do, followed by AMD SATA III chipsets.
It's all to easy to connect a SSD to the wrong SATA ports, people do that all the time.