Frequency of C++ while loop higher than desired

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Hello all!

First of all, my setup is Nvidia Tegra TX1 board with ROS Indigo.

Today I encountered pretty strange issue regarding the frequency of C++ while loop in ROS. While running a program with loop rate of 100 Hz, I measured the time between iterations with 'clock_gettime' function and it gave me the frequencies that varied around 92 - 107 Hz which seemed a bit too imprecise. Therefore, I started investigating and I created a simple program with a loop like this:

while(ros::ok()) {

    clock_gettime(CLOCK_REALTIME, &time1);
    prev_time = ros::Time::now();

    loopRate.sleep();

    clock_gettime(CLOCK_REALTIME, &time2);
    dt = ros::Time::now().toSec() - prev_time.toSec();

    time_between = ((double)time2.tv_sec*SEC2NANOSEC + (double)time2.tv_nsec)
                     - ((double)time1.tv_sec*SEC2NANOSEC + (double)time1.tv_nsec);  

printf("frequency measured with gettime is %f Hz\n", 1/(time_between/1000000000));
printf("frequency measured with ROS functions is %f Hz\n", 1/dt);
}

The desired frequency was set to 200 Hz. I ran the program and for some short periods of time measured frequency was correct (200 - 202 Hz) but at some point I observed that there was a jump in frequency values to more than 250 Hz. As you can see, I measured it with two different methods to be sure that the result is correct. Both clock_gettime and ros:Time based results are the same. What is puzzling is that ros::Rate is based on same timing functions as ros::Time that I used for measuring the actual loop frequency and still the results are not consistent. Also this sudden jump in frequency error is interesting.

What is more, the problem might be hardware related as I tried running the same test program on my PC with ROS Kinetic. It gave correct frequency values...

Any ideas what might cause this behaviour? Thanks a lot in advance.