Sorry,
could you please clarify where the discrepancy is?
What are your sizes for in-memory part, data volume, and backup files?
The reason I am asking that my first thought was that your data volume grew at some point but then it was reduced and the free space was not reclaimed (unlike with log area, you have to reclaim it manually). So in that case we would have in-memory<data volume size (which now has the payload plus the free unused space). But your backup file should theoretically contain ONLY the payload of the data volume.