Quoting Snarkasm
It's just an absolute scale, you sillies. If a RAIDed set of Intel X-25s gets a billion more MB of throughput, it deserves the 7.8.
I'm just assuming that at some point, it begins to weigh access times heavier than bandwidth because there is seriously a wall for mechanical drives on this index. This would makes sense, given the diminishing returns of increased bandwidth for your typical user.
For example, I have an old 7200RPM SATA drive w/o perp recording that scores ~4.3. A single Raptor which is no more than %50 faster in any regard, scores 5.9. Two VRaptors, which offer 4 times the bandwidth of the single Raptor and even better access times, only score 6.2.
I'd love to see the scoring methods.