I've noticed this too. I have my GTX 1080 Ti at +175 core and +650 memory with 120% power limit. It should boost to 2075 mhz but it hits the voltage limit and throttles down to around 2025. Interestingly though the throttle also seems to be determined by the temperature too, although that hovers around 65C with an 80% fan speed.
 
Just tried dropping the offset to +125 since that was the difference in the throttling, which should yield a 2025 mhz boost but the voltage limit comes into play and throttles it down to 1987mhz. All I can can take from this is to get the highest speed possible and if the voltage limit is a factor then let the card throttle as needed.

WeGone
Interesting, I have not messed with the Mem yet. 

Get the core as high as you can, then see what you can get out of the memory. With the +650 offset on the memory alone I managed to get an extra 2 FPS on avarage @ 4K. I might be able to squeeze a bit more out of it because I was going up in 50mhz increments, but I suspect the results would be negligible.