Hi Tdricoll, 

There's a few reports you can run to compare the figures. 

1. PAYG withholding report for the period in your payroll section

2. Journal report for payroll journal in the same period

If your figure in 2 is bigger than 1, then it is likely that some of the journal exported from your payroll didn't carry the right payroll tax code, hence it's not being picked up by your IAS correctly. 

Please let us know how you go in comparing the reports above, and if you require additional assistance on this. Thank you.