Why does the amount of impressions served in the report not match the number of impressions served in the data elements report?

What to do when the impression count doesn't match

The performance report will show every impression that matches a data element.  So if a user matches on three different data elements but is only served a single impression, the data elements report will show 3 impressions, one for each of the elements they matched with. 

That is why the totals do not add up or match the totals on the other tabs in these cases.  Because of this, everything in the data elements area should be taken as relative performance, not actual performance.