I am guessing that the reason for your problem is due to a date difference. The Supply Inventory report will always print as of the Working Date that you are using and the Adjust Supplies will display as of the last time that you entered something for the supply. To check this set your working date to 12/31/99 and then reprint your supply report. It should then match the adjust inventory figure.
To find out if you have a transaction with a bad date, go to the General Ledger and select Accrual Reports and select the Trial Balance. Find the account for the supply inventory and click on the yellow folder button to the left of the account. You will see a listing of the various supplies that you have. Click on the button next to the supply in question and you will see a list of all the checks, farming actions and adjustments for the supply. See if any of these have a bad date (such as a date of 12/31/97 instead of 12/31/96). You can then void the transaction with the wrong date and re-enter it. Or if the transaction is a purchase, you can use the Edit Purchase Date feature to change the date.