Contact lens prescriptions can be different from glasses prescriptions for a few reasons:

 

• The optical center of a pair of glasses is usually located about 12mm away from the front of the eye but a contact lens sits directly on the eye. For some people, this seemingly insignificant distance can actually change the optics of the prescription (In general, higher prescriptions are affected more than lower prescriptions).  Since the optometrist measures your prescription approximately 12mm away from the eye, the contact lens prescription will often need to take this into account.
• Another reason there is a difference is due to astigmatism.  Astigmatism basically means you need a different prescription in one direction (ie: horizontal) than in the other direction (ie: vertical).  If this difference is small, the tear film between the contact lens and the cornea can correct the astigmatism. If it is a larger amount, the contact lens needs to be calculated for rotation and fit to compensate for the astigmatism.  Glasses sit predictably on the eye (the ears and nose help the glasses stay stable in a predictable spot), so the prescription does not need to be adjusted for tear film or rotation.