Why do data set results differ when adding/removing output columns with BIRT?

For some custom reporting, I'm trying to build some data sets based on WORKITEM_CHNGS, but I'm seeing unexpected behaviour when defining my data sets in BIRT. When I change which columns appear in the output (in Fact Information), the actual number of results changes (in Preview Results). This issue is likely related to this other old, unanswered question: BIRT question - Data Set acting strange. I assume that either there is some essential knowledge I am missing, or potentially this is a bug with the application or data. I am using version 4.0 of RTC.

Here is an example of some unexpected behaviours; all examples are filtered by a specific iteration.

With the following output columns, I get 132 records (which I assume is a correct count given the iteration). I cannot remove any column (i.e. use 5 columns) without the results set becoming zero; however, I can replace a column with a different one (e.g. STATE_NAME to STATE_GROUP) and get the 132 records. Is there a minimum number of columns required to define a data set?

WI_ID
PROJECT_AREA_NAME
ITERATION_NAME
CONTRIBUTOR_NAME
WI_TYPE
STATE_NAME

On a similar note, if I add a column to the above set (i.e. seven columns), my record count decreases. For example, adding STATE_GROUP to the above set gives no results. I have also seen combinations that produce subset results, e.g. a result set of 7 (from the 132) with the above columns plus DATE and STATE_GROUP (i.e. eight columns). What is going on here?