Different total word counts delivered by Report Center reports Throughput_Words and Word Count by Project and TM Match Level for the same period of time

In the Report Center, Word Count by Project and TM Match Level  gives me a different number of total words for a given period than Throughput_Words (Throughput_Words is 33% higher). I assumed this was because Throughput_Words drew data from all projects, whereas Word Count by Project drew on some smaller subset (maybe completed?). When I tested with a data range of just one day, however, Throughput was empty and Word Count included multiple projects, so just the opposite of what I was expecting. Any ideas?