I think that is has to do with what frame rates you can get at certain settings. 6000 isin't always better than 3000. You can score 3000 with every detail maxed out or on a different pc you can score 6000 with everything set to the lowest.
I guess it's just a formula that the creator's came up with. So every part of the equation let's say is a variable. After the test is run they substitute the variable with a number according to the results of the benchmark.