why odom values are continously increasing even if my robot is not moving in simulation environment ?

edit

it increases from .0.0123 even if my robot stop

Edit: thanks for your response. I am using a normal gazebo environment you can launch this using gazebo_ros gazebo . i am using the two-wheel robot and some python scripts to measure the distance traveled by a robot all those script files i uploaded below distance is for measuring the distance traveled by robot and movement script used to move the robot to desired distance. there are basically two different scripts ----------------------------------------------------------------distannce--------------------------------------

#!/usr/bin/env python

import rospy
import math
from nav_msgs.msg import Odometry
from geometry_msgs.msg import Point
from std_msgs.msg import Float64

class MovementDetector(object):
    def __init__(self):
        self._mved_distance = Float64()
        self._mved_distance.data = 0.0
        self.get_init_position()

        self.distance_moved_pub = rospy.Publisher('/moved_distance', Float64, queue_size=1)
        rospy.Subscriber("/odom", Odometry, self.odom_callback)


    def get_init_position(self):
        data_odom = None
        while data_odom is None:
            try:
                data_odom = rospy.wait_for_message("/odom", Odometry, timeout=1)
            except:
                rospy.loginfo("Current odom not ready yet, retrying for setting up init pose")

        self._current_position = Point()
        self._current_position.x = data_odom.pose.pose.position.x
        self._current_position.y = data_odom.pose.pose.position.y
        self._current_position.z = data_odom.pose.pose.position.z

    def odom_callback(self,msg):
        NewPosition = msg.pose.pose.position
        self._mved_distance.data += self.calculate_distance(NewPosition, self._current_position)
        self.updatecurrent_positin(NewPosition)
        if self._mved_distance.data < 0.000001:
            aux = Float64()
            aux.data = 0.0
            self.distance_moved_pub.publish(aux)

       # elif self._mved_distance.data>2.23:

           # self._mved_distance.data=0.0
           # rospy.signal_shutdown("quit")



        else:
            self.distance_moved_pub.publish(self._mved_distance)

    def updatecurrent_positin(self, new_position):
        self._current_position.x = new_position.x
        self._current_position.y = new_position.y
        self._current_position.z = new_position.z

    def calculate_distance(self, new_position, old_position):
        x2 = new_position.x
        x1 = old_position.x
        y2 = new_position.y
        y1 = old_position.y
        dist = math.hypot(x2 - x1, y2 - y1)
        return dist

    def publish_moved_distance(self):


        rospy.spin()
        """

        Loops untils closed, publishing moved distance
        """




        # spin() simply keeps python from exiting until this node is stopped

if __name__ == '__main__':
    rospy.init_node('movement_detector_node', anonymous=True)
    movement_obj = MovementDetector()
    movement_obj.publish_moved_distance()

-----------------------------------------------movement-----------------------------------------------------------------

#!/usr/bin/env python

import rospy
from geometry_msgs.msg import Twist
from std_msgs.msg import Float64

def my_callback(msg):

    distance_moved = msg.data

    if msg.data < 2:
        move.linear.x = 0.1

    if msg.data>2.0:
        move.linear.x = 0.0
        if msg.data > 2.01:
            msg.data=0.0
            rospy.signal_shutdown("quit")




    pub.publish(move)

rospy.init_node('test_movement')

sub = rospy.Subscriber('/moved_distance', Float64, my_callback)
pub = rospy.Publisher('/cmd_vel', Twist, queue_size="1")
move = Twist()

rospy.spin()