Piano Analytics is based on our new data factory that allows you to take advantage of all analyses with a "Unique Visitors" scope.
As you will have understood, you will be able to make a noticeable difference between our old data factory and our new data factory for this "Visitors" metric.
This is simply because in our new data factory we are now using only unique visitors, so the "Visitors" metric in Piano Analytics is indeed the equivalent of Unique Visitors in the Analytics Suite 2.
All the tools in the Piano Analytics are using this notion of "Visitors" (i.e. "Unique Visitors") by default.
You can find all the changes that have been made to our metrics in this article: Changes in metrics in the Piano Analytics engine