All these documents were for inside usage of NKVD and other needs. They wasnt supposed to be ever available to the public. They wre unveiled not long ago during massive anti-soviet actions in our country durin Yeltsin rule. Noone edit or hide all these shitloads of dusty papers, lol barley even many ppl bother to read them... You know its boring... Better take some book with catchy name read it and think that you know all shit about it. Or even better not to read book but find some "witness" and listen his cool story, ah, and even more easy is to watch some intersting movie which tell you the truth for sure. All those documents you can compare with other sources and known facts and in the end you will get more or less objective picture. But why bother to do it? Its always easy to believe in things that believe majority among you. Without any odd questions.
Seriously, I wrote my dissertation based from archival research and my sister is an archivist by profession. In that I used newspapers, census data, personal letters, parliamentary meeting records, company financial records ect all dating from the 1800's. I would have failed my dissertation if I didn't use varying sources. I know just how boring it can be, how frustrating it is when information doesn't match up. But equally it's worth it when you turn up gems of information. As others have laughed at, I doubt you yourself have gone and looked at such records.
Where do you think many of those books with catchy names get the research for their books any how? Much of it is archival. Any historian worth an inch will have done some fairly extensive first hand archival research, particularly if they are writing a book and often they will list their sources in a referenced bibliography, meaning you yourself can go and back check it if you like. I did all that and that was for an undergraduate University dissertation for christ sakes.
Finally, you think NKVD documents are going to be open and plain truthed and 100% correct? It wouldn't surprise me if fuck loads of them had been destroyed previously or if they had strict measures for what they actually recorded. Most Government institutions are very careful about what they record on paper regardless of whether they are private documents or public. Unfortunately for us they aren't always stupid. That is why variety in sources is so important, including books as background research and witness statements as well. Anything can be a source of information in history, and you have to look at them all to try and gain a full picture.
Basic historical research methods lesson over for you.