[j7ake]

Although it's unfortunate to say, but when one makes a big scientific journal, the inclination is to go to one of the top journals (Nature index) before going to journals outside of the listed journals. The reason behind this is that getting a paper in one of those journals is directly related to academic payoff (prizes, promotions, grants). Viewed from this payoff perspective, reason behind this Nature index isn't so unreasonable.

This Nature index would be equivalent to counting the number of papers accepted into the top computer science journals. So yes this index ignores the computer science discipline.