I've recently come across a similar issue and started experimenting with different combinations. I have no idea why there is a difference but I have come to find using a HDMI cable with RGB set to limited is even easier on my eyes than using a displayport cable with RGB set to limited. Not sure what the difference is, or what my graphics card might be doing differently, but the screen is a little more washed out like this and for some reason it allows me to use it all day long. I made sure to check the settings for the monitor with both cables and they are exactly the same. Only difference is with this monitor you can select full or limited RGB (at the monitor itself) if using a HDMI cable - in this case I have it set to auto, which still uses full. I plan on seeing if this works the same on another monitor when I get the time.
Specs of what I'm using if it's a help to anyone:
Monitor: BenQ 2700u
Graphics card: 970GTX
Windows: 10 Version 1909