[TorpX]

I recently finished reading LUCK OF THE DRAW, by Capt. C. Kenneth Ruiz. In it he comments on JANAC assessments:

Time and again during the war, the submarine fleet did not receive credit for sinkings and damaged ships for a variety of reasons, including interservice rivalries. The official U.S. summary, The War Against Japanese Transportation, was prepared and published by the U.S. Strategic Bombing Survey. That alone should indicate why submarine sinkings were not always carefully documented. The postwar Japanese records left much to be desired and were of little help in documenting sinkings. The Joint Army-Navy Assessment Committee, which established credits for sinkings and damage, was dominated by the Strategic Bombing Survey report, though there was input from the Submarine Operations Research Group.

[emphasis in original]

While other authors made critical comments about the matter, these are the sharpest I've come across. They strongly suggest that the Silent Service was short changed and sinkings were, in fact, significantly undercounted.

While both Blair, and Roscoe seemed to view JANAC figures with a degree of skepticism, they nevertheless relied on them for their histories. Possibly, they did not see any alternative.

I wish I knew more about the details of how the figures were compiled.