I don't know what you are looking at, but this is likely a confusion of the two types of restaurants in the same building with Wolfgang Puck's name on them.
First, "signature" does not mean "2 credit dining plan restaurant" - it's the other way around. "Signature" in Disney speak means "upscale," not "2 credits on the dining plan." There are signature restaurants that are on the dining plan and cost 2 credits, and there are also signature restaurants that are not on the dining plan at all.
As far as Wolfgang Puck goes - there are two Wolfgang Puck restaurants in the same building. The one on the first floor is Wolfgang Puck Cafe. It's not a "signature" restaurant and IS on the dining plan, costing one TS credit per person.
On the second floor is Wolfgang Puck's Upstairs Dining Room. This is a separate restaurant from the downstairs Cafe and is considered a signature restaurant. They don't accept the dining plan at all.
I'm guessing you saw a reference to the Upstairs Dining Room on the official WDW website, which would say "signature." The website does not do a good job of distinguishing between the Cafe and the Upstairs Dining Room because they have one page that covers both restaurants, although they are separate restaurants. Just one of the many problems with the WDW website.