The accuracy of the optometry data is related to the expertise of the optometrist. I think you should go to regular hospital to undertake optometry for avoiding data mistakes.
The optometry data is related to your mental state and eye condition at that time. The error of optometry is often large when the eyes are tired or your spirit is poor. I suggest that you should not use your eyes frequently before you have an optometry and pay attention to rest.
How long is the interval between your optometry? If the interval period is long, the data will certainly be different because your eye condition is constantly changing.