The problem is we have a lot of bad textbooks in Russia, especially books that involve historical events. History books were rewritten several times,
before the communism, during it and after. We have a saying "Russia is a country with unpredictable past".
You rarely can track a source of information about some historical event, one book references another in distorted way and so on, like a broken phone.
Myths go from one book to another with no ability to track down set of facts from witch historical claim has been made.