If you are looking at RDS, ElastiCache, Redshift service items, this is likely due to reservation apportioning.

For these reservation types, AWS writes a single line for the complete committed amount at the start of the month (you can see this bulk charge if you change the reporting to a daily interval).  The line for the committed hourly fees never changes and does not burn down throughout the month like AWS does for EC2 RIs.  To provide the same visibility into RI utilization that AWS provides with EC2, CloudHealth apportions the used cost from the "unused RI" service into "compute".  However, the apportioning is only within the chart and we don't edit the line items from the bill that show in the drilldown.