My Periodic Financial Report is showing more contributions than income or more income than contributions.  The amount is approximately equal to one Sunday's offering.
What could be causing the discrepancy?

This is usually not an error but simply a product of how dates fall at the end of the month. Let's say that the last day of the month is Sunday, April 30. Contributions are posted on the 30th and will show up on the April report. The deposit will not be made until at least Monday, which in this case, would be May 1. The income (or deposit) will appear on the May report. Therefore, it may appear that the income and/or contributions are incorrect for the months of April and/or May.