Generally, if the Grand Total is changing when any drilldown happens, I would say there are some logic issues between the relationships of different characteristics.
You can also play around with Aggregation settings and Calculate Result rows as in the Query.
As mentioned by Suman just see that you can carry out aggregation setting and see if summation,average might help..
also check if any exception aggregation would require..
see if calculate result rows as in query..there are again various options..
Try to recreate the conditions again with new set of free char etc..
Thanks and regards