A non-Russian speaker is going to be confused when the same character is referred to as both Alexander and Sasha, for example, and will think they're different people.
(FWIW, it lists: Sasha, Sashka, Sashulya, Sashenka, Sanya, Sanechka, Sancho, San, Shurik, Sashunya, Sanyusha, Sanyok. I myself have heard native Russians use Sash - should be written as Сашь -, and e.g. Mish - Мишь -, which is a similar "lazy" conversational short form for Misha/Mikhail.
I've learned some Russian, and once you start sensing the endless magic they can do with verb prefixes and sufixes, you realize what a versatile language this is. Somewhat the same counts for first names, I guess.)
Note: these are written without soft sign a the end: Саш, Миш.
This is a "meme" nickname that 95% of people called Alexander will get pissed over. It's a pretty old diminutive, but because of the movie a lot of people really don't like it.
There are lots of similar names, and the seemingly random use of full names, first names, last names and nicknames, throws off new readers.
There are also just a lot of characters.
The recall of words you aren't familiar with tends to be pretty poor. This is also visible in how hard it can be to build vocabulary when learning a new language, and how you can completely mix up words at that stage - there's nothing about names that makes this any easier.
Similarly, Dmitri Prokofych Razumikhin is referred to as Razumikhin 95% of the time, then suddenly people are referring to Dimitry Prokofych and I've got to look up who that is.