I just tried a quick Google (as I'm sure you did too) and the only references to "volume leak" I can find that don't concern plumbing are where you have posted this question on the web! That suggests that the authors of the document you read made the term up - in which case they really should have defined it rather than leave you to guess the meaning.
If it's a type for volume leak then it might have something to do with databases even though the 1 or 2 times I've heard someone say that it didn't have anything to do with your reconciliation scenario.
lakeshore if you could provide the source from which you found "volume leak" we might be able to figure it out. However, I think it may just be a term the writer came up with in trying to explain something for which there was no term that he/she knew of.