Keep in mind that Nabokov left Russia as a teenager, never to return. Rand left at age 20 or so.
Rand was younger than Nabokov - born 1905 vs 1899.
Pasternak was born in 1890, and was an eye-witness to what ended up in Zhivago.
Nabokov had the great advantage of being an aristocrat and having been educated in English, for the most part.
Actually both Rand and Nabokov left Russia at around the age of 20 - Rand in 1925 and Nabokov in 1919.