jointed rigid body moved due to gravity

I am experimenting the unity physics engine.
When I have one rigid body jointed with another rigid body , using configurable joint
with the x , y and z motion locked, and the base rigid body being constrained
to a fixed position and rotation,
the jointed rigid body dropped down by a fixed offset due to gravity. The offset is
proportional to the ratio of the mass between the base and jointed object.
It this normal and can it be avoided ?
Attached is the testing project I've written for this scenario.