Gazebo simulations not repeatable

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I have been using an custom obstacle avoidance node in gazebo simulator with Turtlebot. It is based on image and poincloud inputs, and gives an angular velocity in output. I ran my code with two identical simulations (an big simulated room with one box) to study the repeatability of my algorithm. Each time, the trajectories are different. So to verify that it was not coming from my code, I fed the same recorded bag file of images and pointclouds to my code, an it produces exact same behaviour. So the simulator seems to bring some randomness, but I cannot figure out the precise source of the problem since there is not much documentation on how the world, objects and robot are simulated, how the simulated images and pointclouds are rendered. So I need your help, to help me narrow down the origin of randomness between simulations. Does it come from the image and pointcloud generation ? Or from the odometry from a model small inaccuracy ? Here are the parameters I use:

<physics:ode>
  <stepTime>0.001</stepTime>
  <gravity>0 0 -9.8</gravity>
  <cfm>0.000000000001</cfm>
  <erp>0.2</erp>
  <quickStep>true</quickStep>
  <quickStepIters>100</quickStepIters>
  <quickStepW>1.3</quickStepW>
  <contactMaxCorrectingVel>100.0</contactMaxCorrectingVel>
  <contactSurfaceLayer>0.001</contactSurfaceLayer>
  <updateRate>1000</updateRate>
</physics:ode>

<geo:origin>
  <lat>37.4270909558</lat><lon>-122.077919338</lon>
</geo:origin>

<rendering:gui>
  <type>fltk</type>
  <size>480 320</size>
  <pos>0 0</pos>
  <frames>
    <row height="100%">
      <camera width="100%">
        <xyz>0 0 10</xyz>
        <rpy>0 90 90</rpy>
      </camera>
    </row>
  </frames>
</rendering:gui>


<rendering:ogre>
  <ambient>1.0 1.0 1.0 1.0</ambient>
  <sky>
    <material>Gazebo/CloudySky</material>
  </sky>
  <grid>false</grid>
  <maxUpdateRate>10</maxUpdateRate>
</rendering:ogre>

<model:physical name="gplane">
  <xyz>0 0 0</xyz>
  <rpy>0 0 0</rpy>
  <static>true</static>

  <body:plane name="plane">
    <geom:plane name="plane">
       <laserRetro>2000.0</laserRetro>
      <mu1>50.0</mu1>
      <mu2>50.0</mu2>
      <kp>1000000000.0</kp>
      <kd>1.0</kd>
      <normal>0 0 1</normal>
      <size>51.3 51.3</size>
      <segments>10 10</segments>
      <uvTile>100 100</uvTile>
      <material>Gazebo/GrayGrid</material>
    </geom:plane>
  </body:plane>
</model:physical>