I majored in Russian in college (many, many moons ago) so I can only see
Sasha as a boy's nn for
Alexander.
Nns in Russian are a lot of fun. Suffice it to say they love to make the nn as soft and "smooshy" as possible. The "x / ks" sound in Russian is what turns into an "s" sound at the start of
Sasha.
Maria becomes
Masha,
Pavel becomes
Pasha. There's no real formula, just the desire to smoosh.
I prefer the
Sasha spelling because there are strict transliteration rules and that's how a linguist would spell it. (I'm guessing the "ch" spelling is a
German or Yiddish one. But as I say, that's just a guess. Does anyone else know for sure?)