It is unlear how the higher number was arrived at; usually, the decision increasing support will state the grounds and also include a copy of the actual calculations of the court. If you are already in front of a support magistrate on your order to show cause, then you need to present your income by way of tax returns and W-2's for the entire period in question. You may also consider engaging an attorney on this application.
Please be advised that this answer shall not constitute legal advice, or create an attorney-client relationship.
How long has this order been in place? Were you ever served with the support order? You can hire an attorney and discuss submitting a motion to vacate the default order. Also, if you were never served with the Order, you can argue that the time to file objections has not run out, and you should be able to do this as well. I certainly recommend hiring an attorney to do so for you.
Make a motion to vacate the default based upon not being served properly. If necessary you can subpoena the person who allegedly served you. Also, they will need to calculate the amount of child support based upon your total income utilizing both your tax return from last year and the most recent pay stubs which includes the year to date amount.