Don't forget that Harry Hinsley didn't have to rely on records entirely - his memory of what he got up to at Bletchley helped to inform his writing somewhat... (as did the input from former colleagues).
IIRC, there was a slight element of self-censorship in some parts of the work to avoid revealing matters which still had a degree of currency and which were deemed sensitive (Gordon Welchman lost his US security clearance [he moved there in the late 40s and became a US citizen] after writing his memoir of Hut Six in 1982).
The history may be incomplete, but not quite for the reasons one might assume...