ofxBox2dPolygon bounces so fast

I’m working on a project with ofxBox2d.

It seems that ofxBox2dPolygon sometimes bounces so quickly though I set bounce in setPhysics().

ofxBox2dPolygon poly;
vector<ofVec2f> vertices;
// set vertices
poly.setPhysics(4.0, 0.4, 0.1);

Then it bounces off the ground so fast and flies away.
Does anyone have an idea? Or is it because it’s polygon?

I’m running oF 0.8 on Mavericks.