Why Does Prescription of My Glasses Vary at Different Clinics?

So how could it be, that the same patient could get two different prescriptions from two different eye doctors, even with eye exams on the same day? The truth is, there is no ONE set of prescriptions that work. Human eyes can adjust and adapt to small differences in prescriptions because it is actually the brain that is interpreting the image that it is given by the eyes. The human brain wants to see as clearly as possible and will try to work through any prescription that is put in front of the eyes.
If you’ve ever tried another person’s glasses on, you may have noticed that the images are very distorted or blurry; or worst, you may have even felt sick to your stomach. This feeling is your brain telling you that the prescription is not correct. If the prescription happens to be very close, you may think your friend’s glasses might just work for you. This feeling is your brain adapting to the power in the glasses.

Other Factors
How tired you are, the time of day, health conditions such as diabetes can all affect the prescription the doctor is giving you. Remember that in an eye exam, the doctor is asking you to respond to choices that are presented to you. If you are
tired, can’t focus well or have fluctuations due to medical conditions, your responses may not be accurate. As eye doctors, we need the patient to help us determine the prescription; it is not just the doctor giving you the eyeglass numbers.
Your prescription measured at the eye doctor is a snapshot of your vision at that exact time. If you don’t go to the eye doctor often, it’s highly likely your prescription has changed. The more you test your vision, the more vision data you’ll have to compile the most accurate prescription possible.

How would I determine whether my glasses are right for me?

So, as someone who wears glasses, what can you do if you are not seeing properly out of your new prescription? First, you should give it a few days to see if you adapt. For every change in the sphere, cylinder, and axis, it can take some time for the brain to feel comfortable and re-focus the eyes.

Elements of a Prescription

Essentially, an eyeglass prescription contains three distinct numerical values: sphere, cylinder, and axis.
The ‘SPHERE’ is the number that indicates whether you are nearsighted (myopia) or farsighted (hyperopia). The axial length of the eye determines myopia and hyperopia. In plain terms, the measurement from the front of the eye to the back of the eye. Myopia is written with a (-) sign in front of the power and hyperopia is written with a (+) sign in front of the power.
The ‘CYLINDER’ is the number that describes the amount of astigmatism that the eye has. Astigmatism is a measure of the curvature of the eye.
The ‘AXIS’ is related to the cylinder curvature and shows the direction of where the eye is curved.
These measurements are your eyeglass prescription numbers and are used to make glasses. When glasses are manufactured according to the prescription, there is a tolerance range of how far off the prescription can be from what is written. Generally accepted tolerance ranges are within 0.25D (diopters) of the prescription written for the sphere and the cylinder. For the axis, measured in degrees, it can vary depending upon how much cylinder power there is. Generally, the higher the CYL, the smaller the tolerance for the axis.
Since the visual tolerances are small, that means that eyeglasses must be made very accurately to the prescription.

Source : eyeque.com