Your motherboard does not have native SATA 3 (6Gb/s) ports.
Your motherboard uses a Marvell 9128 controller for the 6Gb/s ports which has a data bandwidth of PCIe x1 at 5Gb/s (500MB/s).
You won't get advertised Read/Write speeds from any current generation SSD you connect to those ports.
That being said, your ATTO benchmark results still should be higher.
Double-check that your drive is connected to port SATA_6G_1 or SATA_6G_2.
See page 2-2 of your motherboard manual for the location of the ports.