Thats what I thought too but this is what happened to me recently. I booked two seats and used my vouchers to upgrade both. Mine booked 5 days back in K class and the other booked one day before the flight in S class.

Upon checking on the day of the flight the higher (S) class which was booked later had cleared and the one booked 5 days before in K class was still wait listed. I am a Gold and the other wasn't even a JP member.

I was quite peed off about this, so this means that Jet does not actually wait list rather they clear upgrades depending on class of booking.