Why does robot behave differently written in SDF and written in URDF with SDF code included in <gazebo> tags

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I have created an SDF model of a robot whose cover has multiple parent links (can't be done in URDF). It worked nicely, moving smoothly when force applied, as expected. Then I took the code for cover in SDF and put it into the URDF code into <gazebo> tags. The model loaded in Gazebo as the SDF model did, but now the joints belonging to the multiple parent cover are shaking without reason and it seem that it takes more force to move the cover. Why does it behave differently? Can I fix it?