It's a limitation of your monitor. A lot of LCD monitors are only 6-bit and can only display a total of 64 levels per color channel. Textured imazges look OK, but gradients will show noticeable banding. You have two choices:
1. Live with it.
2. But a new monitor with a 10-bit LUT and internal hardware calibration such as an Eizo. But they cost a lot of $$$$.