The expressions for Physics properties (such as Sprite.Physics.Density) will pick the values from the first object in the behaviours corresponding family.. rather than the object specified.
So say we have Sprite1 and Sprite2 in the same family, and give that family the Physics Behaviour...
using the expression sprite2.physics.density will return Sprite1's density instead.
Same for elasticity, friction, etc
Here's a cap:
I haven't tested this with other types of behaviour.