[ODE] Joint deformation - why that hapens (how to prevent it)?

[michael kapelko]

Screenshot: http://kornerr.alfamoon.com/pictures/joint_hinge_strange.html
Here two white "planes" are connected with a hinge joint, but the left 
can still be still deformed not along hinge axis. Well, you can see on 
the screenshot that the left one is not perpendicular to the floor, 
that's what I mean.
Why is joint allowed to be deformed that way?

Src & exe: http://irrlicht.sourceforge.net/phpBB2/viewtopic.php?t=17317

Thanks.