Why is my prescription not the same on my lenses and on my spectacles?

There are several reasons to why the two prescriptions don't match. If you have correction for astigmatism in both prescriptions they are almost never the same and that depends on that glasses are made with much smaller steps in astigmatism correction.

If you have a low need of astigmatism correction you will most likely not have any correction for astigmatism at all in your lenses. If you have high myopia or hyperopia the power on the prescription will vary depending on that the distance from your eye to the glass differs from the distance between your eye and the contact lens.