I have a simple report with showing some production data with 4 levels of grouping (Product Family --> Department --> Product Code --> Machine

One of the columns contains the number of hours of downtime allocated to the machine for the given product, etc... This value is divided by another value in the dataset called "Scheduled Hours" to come up with a % of downtime per the hours scheduled for the machine.

Anyway, next to this computed value I have another field that controls the visibility of a bar chart (shown in the same group level). The bar chart is intended to show the Top N reasons for the downtime as a % of their contribution to the total. For example, if the Downtime % of Sched Hrs was 20%, the chart would show the top N reasons totaling up to the 20%. 

The Data Tab for my chart contains the following info:

The Values come from this expression: =Sum(Fields!AvailabilityDownHours.Value)/Sum(Fields!KBScheduledHours.Value, "SummaryTable_Group_Product")

Under Grouping:

Grouping On  "Fields!ReasonCode.Value
FILTER EXPRESSION:   =SUM(Fields!AvailabilityDownHours.Value)     Operator: Top N   Value:  =10

SORTING EXPRESSION:  =SUM(Fields!AvailabilityDownHours.Value)    Direction: Descending

The problem I am having is that if I drill into one chart that only has > 10 downtime reasons, I see all of them. (16 in this case). So it appears that the filter is doing nothing to filter out the 6 I don't want to see.

All the data for this is coming from the same dataset. 

Any ideas on why this is happening?