If your card is running at stock setting after you decrease the clock, then it is probably go boost 2.0 that is pushing it past.

You will need to verify that at 0 offset , the gpu isn't boosting to 1316. If you are getting a boost to 1316, then it is gpu boost 2.0 that is doing that, and not really an issue with precision.

Verify these findings with GPU-z using the sensors and selecting max on the Core clock.

I am guessing you own part number 2983-kr, since it is the only one with a base clock of 1266?

GPU boost 2.0 is an NVidia feature, and the only way around it is a custom bios, which would need to come from someone comfortable with editing the bios itself. It allows the card to boost to highest possible clock that the card can maintain with stock voltage.
 
I personally am back and forth with it.  All of my cards have been stable, but I hate it when I am trying to overclock, as it can sometimes boost higher or lower than your plan.  Other than that, I really don't know what else you could do other than request someone to set up a custom bios for you to help you out.
 
you could try NVidia Inspector, which will give you some control over your card (not really changing gpu boost, but the ability to over/underclock) or maybe even MSI Afterburner, to see if either of those will work slightly better for you.

KBoost is supposed to help when your card is downclocking during older games.  It helps maintain a steady clock, and keeps it there so that your FPS doesn't fluctuate.  This is also good for benchmarking so that the clock speeds do not have to "rebuild" between scenes where the GPU will downclock while waiting for the scene to load.
 
KBoost is not considered the best thing to use for normal every day games that are fairly new.. Maybe something really old, like Riven, Myst, or something like that where the GPU downclocks and drops FPS.  
 
GPU Boost 2.0 is pushing your card (kind of overclocking it) for you, because it is capable of getting to that clock.  If it weren't, it would cause major instability.  
 
my cards all set top run at 1085mhz boost, but they actually all 3 run at 1149, because the card is stable at those speeds.  Each card may react differently, but I got lucky with all 3 being nearly equal. my 760SC are both rated to run at 1150mhz.  One runs at 1150, while the other runs at 1215.  Just luck of the draw in the silicone lottery.
 
This doesn't hurt anything at all, but running K Boost can degrade the chip if you run it all the time.  Imagine it like the engine in the car compared to a GPU.  When the GPU is benchmarking or playing a game, it is at full throttle.  The manufacturer says "your car will go 120mph on the straights guaranteed".  You hit the straight and you get to 135... you don't complain, because you got more than you paid for... so when you take a sharp turn (the point where a scene is loading and the graphics card downclocks.) you let off the gas and apply the brakes a little.  
 
With KBoost, same scenario, but you are holding full throttle even while you are sitting still.  If you wouldn't do that to your car, why would you do it to your hardware?  
 
Now, say you have a really old car, and the only way to keep it running is to keep the throttle pinned to the floor.. this is where you want KBoost.  If you are playing anything new with KBoost, you are essentially forcing your card to wear itself down.