Three pointers I'd like to point out.
Check for dust buildup. CPU and GPU clocks and stability are highly affected by thermals.
Lastly and this is important; Have you considered that other people are also benchmarking their 1080ti overclock and that your 1080ti was not as golden you thought it was? (based on UserBenchmarks)
Also the card auto boosts to 2025 Mhz while it's stock frequency is ~1595 so in a sense it's over-performing what it was rated for. If you send the card back you will be at the mercy of silicon lottery gambling.
My 1080ti FTW3 won't boost to 2025mhz so you're already ahead of my card, (my card caps at 2012mhz on the core (127% power target, modded Vbios)