I noticed the same behavior.
For the monitor connected via HDMI, the refresh rate is 60HZ on all resolutions.
For the monitor connected via DP, 60HZ is the default for 2048*1080, 1080p, 1080i, 720p.
59.88HZ is the default for 2560*1440, 2048*1152, and 1600*900.
But I am not sure whether this...