When your case comes before the judge, he has the option of approving your claim as filed, denying your claim, or issuing a partially favorable decision approving benefits but using a different onset date. Usually, when the judge changes your onset date he will use a more recent date, which will have the effect of reducing your past due benefit amount.
Why would the judge change your onset date? Most often the judge will do this because he has concluded that the medical evidence in your file does not support your original alleged onset date. You can expect this to happen if you alleged an onset date prior to the date you stopped working, or if your onset date was more than two or three years ago.
Some judges will choose a different onset date based on the presence of a medical test, such as an MRI or CT scan. This can be frustrating if, for example, you hurt your back three years ago, but could not afford an MRI until two years ago. Some judges just do not feel comfortable assuming anything, despite the very strong likelihood that your disc issues existed as of the time of your traumatic back injury and did not suddenly come into existence a year after your accident.