Historically, one big item in AWS cost reports that wasn't able to be assigned various costing tags has been EBS snapshots. The reasoning behind this was presumably tied to their differential nature-- it's presumably non-trivial to allocate thousands of very small amounts of data to a cost tag, nor was it a particularly high priority. After all, unless you're doing something strange you're unlikely to be spending significant amounts of money on snapshots.
Of course, AWS is vast enough that many of us are indeed doing something strange! In some cases I've seen snapshot costs rise into the tens of thousands.
As of today, tags propagated to your snapshots now act as cost allocation tags once enabled in your billing dashboard. Combined with a tool (I'm partial to Lambda functions) that propagates tags to from instances to EBS volumes, and then further to snapshots, you get a more accurate cost picture for various environments, business units, and projects.