The better question is, why would expect them to be the same? It is all in the math and on the value that your state places on an exemption. If you are getting a large refund then you overpaid during the year substantially. That says to me that your exemptions did not lower your taxable income in any given pay period very much.
There are plenty of web tools out there to estimate the effects of different numbers of exemptions on your year end taxes. Try one of them so you can properly set them this year.