Earlier quoted context omitted.
I haven't read any Dostoyevsky since high school, and don't remember it at all, but I'd imagine it has to do with nicknames. 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.
I've never understood this particular issue. My mother had it too with Russian literature. Many Russian books have a "cast list" at the beginning to get round this. I don't find it any stranger than William being called Will, Wullie, Bill or Billy; or Robert turning into Rob, Robbie, Rab, Bert, Bob or Bobby; or Elizabeth being turned into Liz, Lizzy, Beth, Betty, Liza, Lilbet etc. I found most Russian diminutives are…
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.