Different values in /cmd_vel/angular/z and /odom/.../angular/z

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Hello. I am new in the use of ROS. I'm using simulation with model p3dx in ROS Indigo and Gazebo 2.x in Ubuntu 14.04 64bits. I'm struggling with the difference between: a) the cmd_vel.angular.z values (value sent to the robot) and b) the odom.twist.twist.angular.z (value obtained from odometry), after stabilize the speed, exacly as link image: link text

In the model p3dx using the differential-drive library, for which the sending angular velocity values (cmd_vel / angle / z). I wonder if this can be solved with configuration or is a platform of error.

Below the library insertion section in "pioneer3dx.gazebo":

  <!-- differential drive -->
<gazebo>
    <plugin name="differential_drive_controller" filename="libgazebo_ros_diff_drive.so">
        <alwaysOn>true</alwaysOn>
        <updateRate>100</updateRate><!-- original = 100 -->
        <leftJoint>base_right_wheel_joint</leftJoint>
        <rightJoint>base_left_wheel_joint</rightJoint>
        <wheelSeparation>0.39</wheelSeparation>
        <wheelDiameter>0.15</wheelDiameter>
        <torque>5</torque>
        <commandTopic>cmd_vel</commandTopic>
        <odometryTopic>odom</odometryTopic>
        <odometryFrame>odom</odometryFrame>
        <robotBaseFrame>base_link</robotBaseFrame>
    </plugin>
</gazebo>