Did they take your voucher in return? If yes, you received the upgrade because there was space in J and that there wasnt anyone with higher status requesting upgrade...

if no voucher was taken, this is an op-up which to me is a rarity on Jet Airways.. only reason for op-up's is that you are a celebrity (smile.. sign..) or flight was completely full in Y and there werent that many elites on the flight as you being silver got an upgrade.. if you did get an op-up on 9W - i'd say go get a lottery ticket that very day... its def 1 in many (million/billion) chance.. IMHO atleast..