Final answer:
Contact lens prescriptions and spectacle prescriptions may differ due to the way they correct vision problems and the advantages they provide.
Step-by-step explanation:
The main reason why a contact lens prescription and spectacle prescription may differ is due to the difference in how they correct vision problems. Contact lenses rest on and move with the eye, providing better peripheral vision and correcting corneal astigmatism caused by surface irregularities. On the other hand, spectacle lenses sit in front of the eye and have advantages such as a fixed distance from the lens and the ability to correct different parts of the lens system producing images at different locations. Additionally, spectacle lenses are used to correct astigmatism and can have a cylindrical correction. Additionally, if the curvature of the contact lens does not exactly match that of the cornea, this can lead to the tear layer having lens-like properties that require adjustment in lens power. Skilled optometrists will adjust the power in contact lenses to compensate for these differences, which is often not necessary with spectacle lenses because they treat astigmatism with cylindrical corrections and have spherical corrections for short- or farsightedness that are the same across all axes.