One reason might be that the items were posted but no fee was charged.  The most common reason would be a member who was opted out from Reg E services, so that when the item was presented we force-posted it but could not assess a fee.  The other reason could be an item that fell within your configured no-fee tolerance, so the member didn't get a fee.

This is because the program that actually writes out those detail records is the same program that posts the fees in the first place. If the fee routine isn't run, then those detail records are not gathered, either, even though the counters themselves are incremented.