I've seen them too, and usually on the older NGC slabs when these first started rolling out. I think these were a sales tactic by an early distributor applied to the less corroded pieces. It doesn't correlate to the 1-70 scale. I wouldn't be talked into a high price because its there.
With shipwreck coins, eye appeal matters and to each their own. I've seen this sticker on plenty of miscolored pieces I thought were ugly that happened to have less corrosion. I own a few heavily corroded El Cazador that I got cheap cause I thought they still looked cool.