I think the issue is that Suprnova uses a very short "N" value for PPLNS calculations on ZEC. This makes the results extremely variable. If the N value were increased, payouts would be much less variable, but it would take longer to get your coins.

Anyway, it's very possible for your shares to be worth nothing with PPLNS. It's also common for them to be worth 2 or 3x as much (or more). It all depends on luck.