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You're not using the msg variable that is passed into the callback; you're trying to get the ticks from a global variable instead.

Your callback should probably be more like:

void WheelCallback(const mastering_ros_demo_pkg::encoder_msg::ConstPtr& msg)
{
  //tijd van de meting 
  current_time_encoder = ros::Time::now();

 //verschil in ticks tov vorige berekening 
  deltaLeft = msg->ticks_links - _PreviousLeftEncoderCounts;
  deltaRight = msg->ticks_rechts - _PreviousRightEncoderCounts;