formula claculation in query is not summing correctly when details are remo

have a report that totals 2 columns together and is at the work center level. when I remove the details the totals are wrong but when I show them it comes over correctly. Here is the scenario

work center column a column b sum a+b

XXX 2 2 4

the details however bring in 3 and 5 & 8 for columns respectively. Why would the work center level be wrong. do I need to do anything special in my formula?