Are old hashrate numbers not to be expected as time goes on?

I'm aware "difficulty" is a thing, but in my experience that usually just affects profit numbers, and not raw hash. Is this not the case with KAWPOW?

I have a 2080 Super that can barely scrape out ~18MH on an overclock when apparently I'm supposed to be expecting 30-40.

This is what it looks like. I'm not even stable at those settings but I saw them getting thrown around as best case scenario.

Following NiceHash's guide gets me like 60% of what they claim I should be at.