Why would Quicken incorrectly add up my list of checks / debits?

My reconciliation is off by $500 and I couldn't find the error even after going through my statement line by line. I keyed in the amounts into Excel and noticed that I had 51 transactions even though Quicken showed 50. I reset the register and tried again. It then showed 51 transactions but the list of checks / debit transactions looked the same. The total shown in Quicken is $500 less than the total of the statement (I double checked in Excel). I then manually added up the transactions in the reconciliation window and it's off by $500!. Has anyone got a suggestion as to why the sum of the checks / debits displayed in Quicken is incorrect?