Still, they report excessive alcohol concentration in 4-9% of fatal
accidents, which is way higher than the Nall Report's estimate of 1.1% (for
alcohol and all other drugs combined). I wonder what accounts for the
discrepancy.

Decay, actually. Leave a dead body to sit for a while, and one of the
decay products will be alcohol. Since bodies in aircraft crashes are
often not recovered immediately, this is one of the problems with the
toxicology. It's well known, and the NTSB is aware of the problem, but
certain organizations with an axe to grind aren't interested in giving
readers the complete story. The Nall report is based on NTSB probable
cause, which takes such factors into account. The study probably uses
raw toxicology data.

Michael