These reports are calculating different totals - the Appeal Performance Report counts pledge amounts that were given, but the Revenue Detail report doesn't count pledges - it counts the payments that are made that are tied to the appeal.  

For example:

Create an appeal and make a $1,000 pledge towards it.  Then, make a $100 payment towards that pledge.  After saving the payment, edit the payment and clear the appeal.  If you run the Appeal Performance Report for that appeal, it shows $1,000 since that’s is how much the appeal actually brought in.  The Revenue Detail Report just lists specifics transactions that match the criteria.  Since the pledge has the appeal but the payment doesn’t, only the pledge is included in the report.  The pledge’s balance is $900 so that is the correct amount to display.