I'm not sure if this has already been mentioned, but are you pulling data from more than one table/query for the query that the report uses as a source? You could have an instance where the two tables are creating more than one unique record for each row in your report. Say, for instance, that you have a report by month listing total department hours worked and paychecks paid. The query pulls from two tables, one that shows each person's hours, and another that shows total paychecks paid per month.
Your report would show:
Month Total Hours Paychecks Paid
1 100 1000
2 100 1000
3 100 1000
But, since you are pulling from two tables, and you enter paychecks paid as a total for the month, but add sum the hours from several rows in a table, your data in the query shows something more along the lines of:
Month Total Hours Paychecks Paid
1 20 1000
1 40 1000
1 40 1000
2 20 1000
2 40 1000
2 40 1000
3 40 1000
3 40 1000
3 20 1000
I had a very similar issue yesterday, and even though the report showed numbers that should add to, in this case, $3000, the sum/runningsum would add up to $9000. I had to split up the queries, total them seperately, and then use another query to bring them together. I don't know if this is what you have, but I hope it helps.