What they show, are "average" FPS. But average as how? Different games use GPU at different amounts. E.g you'll get far higher FPS in Cities:Skylines than in e.g Crysis. Also, it depends what the rest of the system is. Running Core i7 gives overall better performance than running same gen Core i3. Heck, even two identical systems will not get the exact same FPS amount.
And on their page, they state:
Our GPU benchmark results are measured against the user preferred gaming resolution and game quality settings. We have sophisticated algorithms that have been carefully designed to produce 90% accurate estimates of gaming performance based on analyzing over 70,000 benchmark tests. Although graphics card performance varies greatly among different games, this comparison chart provides a general ranking index for graphics cards.