tannins are part of the defense system of a plant.. They protect the plants against a number of threats.  Maybe there is something about the area in mexico where it grows that makes the plant increase the ammount of tannins to protect itself. I guess the tannins are mostly against biological threats (whether its animals that can chew on the plant or insects or a number of infections), but otherwise one possible difference between Dreamland's northeast where mimosa comes from, and mexico, is the temperature. In the northeast Dreamland the temperatures never get low, its always hot. Maybe in mexico it gets colder, and this could influence.. 

Or maybe its just the supplier you are getting from..