Synchronization of RGB and depth topics in ASUS Xtion

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I am checking whether the /camera/rgb/image_raw and the /camera/depth/image topics of the ASUS Xtion camera are synchronized. For that purpose, I have a simple script with two different subscriptions in the topics and each time I print the difference of the timestamps. The code is shown below.

#include <ros/ros.h>
#include <sensor_msgs/Image.h>

long double rgb_time, depth_time;


void callback_rgb(const sensor_msgs::ImagePtr& rgb){
    rgb_time = rgb->header.stamp.toSec();
    ROS_INFO("Difference between rgb and depth: %Lf", abs(rgb_time-depth_time));
}

void callback_depth(const sensor_msgs::ImagePtr& depth){
    depth_time = depth->header.stamp.toSec();
}


int main(int argc, char** argv){
    ros::init(argc, argv, "queue_points");
    ros::NodeHandle nh;

    ros::Subscriber sub = nh.subscribe("/camera/rgb/image_raw", 100, callback_rgb);
    ros::Subscriber sub2 = nh.subscribe("/camera/depth/image", 100, callback_depth);

    ros::spin();
}

The problem is that the log shows that the timestamp difference gradually increases from 0 to ~0.033 and then it resets to 0. I have checked using rostopic hz that both topics are being published at ~30Hz. I know that the good practice when data fusion is of concern is to use Time Synchronizer. However, shouldn't this approach show if the messages are synchronized? My question is whether this behavior in the timestamps is what actually happens or whether it is misleading.

I am using Ubuntu 18.04, ROS Melodic and the camera drive is the ros_astra_camera. Thank you a lot in advance and let me know if any more information is necessary.