Result screen for normal non-stress test runs shows how good your score is compared to all other users with the same CPU and GPU.

Legendary score is in the top 1% of results
Excellent score is in the top 15% of results
Great score is in the top 30% of results
Good score is in the top 60% of results

- in comparison to results made with same CPU and GPU.

Edit: Important: This is not the same as online result page showing your score placement vs all 3DMark results ever submitted with that test. Those values are often 95%+ for modern gaming PCs as that indicator compares against all 3DMark results ever submitted, including random office PCs or gaming PCs from 5+ years ago, so surprisingly any modern GPU tends to be at the very top

The achievements do not use that value. They instead compare your result vs. all results from users with same CPU, GPU and number of GPUs (in case of SLI/Crossfire). You can use 3DMark search to see the score distribution for your hardware combination by just going to www.3dmark.com/search and choosing the test you are going to run and your CPU & GPU model and the graphs in the top show score distribution and average score for the combination. Beating that average should comfortably get "Good" rating.

This was already added a while ago to the UI and the new achievements just piggyback to this. A qualifying score must be run at default settings.

And yes, the top results tend to be from overclocked systems. A stock system with good cooling and no background tasks to slow things down usually hits a "Good" score, but the margins in the most common results are fairly small especially as most modern GPUs are very sensitive to temperatures. Just manually setting GPU fans to run at 100% during a test can easily be seen in the score.

And at least right now higher achievement will not trigger the lower ones, by design. If your OC is "too good" and you want the lower ones, you get to do some tweaking to hit the lower score ranges for the lesser versions.

Update: Known issue right now that if the combination of hardware is very rare (less than 100 results), the system cannot give the rating at this time. We're working on a fix to this. For SLI / Crossfire results note that this counts as 100 results with that GPU count.

Update2: Above known issues should be solved with 2.20.7290