To echo Darth Baiter's comments above....

My guess (based on how some of the past selective gear work has gone) is that these values, which stipulate McNary survival, are likely based on only a subset of the fish handled - those that they tagged that passed Bonneville. So, these values should probably be tempered with some more context. Not that they aren't likely pretty good, relative to other gears.

Unless Doc was there, at the trap, to help them ID the stock/run of fish and perhaps even the tributary/hatchery of origin at the time of tagging?

(good fun intended)