External monitor's (DELL U2415) refresh rate locked at 59.88 Hz (NTSC)?

I bought a DELL U2415 monitor and connected it via a MiniDP to DP cable that came with the monitor. Surprisingly the refresh rate of the monitor was locked at 59.88 Hz (NTSC), unlike a U2414H connected via HDMI, which has its refresh rate configurable as either 50 Hz or 60 Hz.

59.88 Hz looks like a strange number to me as well as the NTSC that came with it. Is it possible to change the refresh rate, e.g. by using some other software or using HDMI cable instead of DP cable? Is there any potential downside to this refresh rate? I feel the colors slightly faded but maybe that isn't related to the refresh rate per se.

I have the same problem with my MPB 2012. I have a DELL P2418D monitor and connected to Thunderbolt to Display Port. After hard time searching a solution I discovery that cable is the problem. My solution was buy a HDMI adapter from ICZI that supports 4k but my MPB 2012 need 2k and works very well with 60 Hz.