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

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Hello. I am new in the use of ROS-Gazebo simulation. 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: cmd_vel_vs_odometry

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":

<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>