How to calculate RPM correctly using encoders?

edit

I have optical encoders that read the ticks and publishes the tick count everytime when there is a new tick:

 def tick_callback(channel):
    global tick
    global pub
    tick += 1
    pub.publish(tick)

class EncoderNodeLeft:

    def __init__(self,encoder):
        self.tick = 0
        self.encoder = encoder
        self.rate = rospy.Rate(100)
        GPIO.setmode(GPIO.ASUS)
        GPIO.setwarnings(False)
        GPIO.setup(encoder, GPIO.IN, pull_up_down=GPIO.PUD_UP)


    def run(self):
        global tick
                GPIO.add_event_detect(self.encoder, GPIO.RISING, callback=tick_callback)
        while not rospy.is_shutdown():
            self.rate.sleep()

But the problem arises when I need to count the RPM and in order to do that, I need a current tick, the last tick and the time difference. The difference between current and last tick is always 1:

void MotorRPM::encoderCallbackRight(const std_msgs::Int64::ConstPtr &msg) {
    std_msgs::Float64 rpm_msg;

    // get the time difference
    ros::Time now = ros::Time::now();
    ros::Duration time = now - lastMessageTime;

    int rightDifference = msg->data - lastRightEncoder.data;
    //ROS_INFO("The tick: %d, the lastData: %d", msg->data, lastRightEncoder.data);

    rpm_msg.data = rpmFromEncoderCount(rightDifference, time.toSec());

    // publish all the time
    rpmPubRight.publish(rpm_msg);

    lastMessageTime = now;

    lastRightRPM.data = rpm_msg.data;

    lastRightEncoder.data = msg->data;
}

Am I doing something wrong? Because I always have a difference of 1 I get incorrect RPM