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 formulaic so I picked them up fast.