Frequency Error

Hi,

I am using the MetaWearRPRO sensor for data recordings. I had a general question about the error in gathering data. I am recording data directly on the device and transferring it over to a tablet after complete data collection. The data is being sampled at 100 Hz. I plotted the data and tried to match it with actual time stamps; however, there is clearly a deviation of the data as seen from image below.

I have read the data sheet of the Bosh BMI160 sensor and see the sampling error rate of 39 micro-seconds; however, the error seems have a 1 extra reading per 100 samples making the frequency rate at 101 Hz rather than 100 Hz. The sampling rate error seems to be constant and I can adjust it accordingly; however, I wanted to know you know the reason for the error on a hardware or software level? Is there a sampling rate tolerance that I am unaware of? (I already read through most of the forum and couldn't find the exact same query that I am posting)