Benchmarks

I downloaded the Xbench just to see how fast was my iBook, wich scored 99.71, but after checking some of the scores at Xbench's page i got some questions:

- How is it possible that some machines with 768mb RAM got better scores than machines with 1256mb RAM? does it has to do with the brand? in that case wich brand is the better?
- What is the "Apple read/write" that reads on the top scores instead of (what i think is) the hard drive's brand?
- I got a bit more than one point of difference using Shadowkiller, is there some other software/driver i can use to improve the performance?
- Any other advice, help, suggestion about this?

Thanks!