AWS charges a $2 per hour fee whenever any Dedicated Instance is running within a region. More information can be found regarding this charge here: Amazon EC2 Dedicated Instances
If multiple Dedicated Instances are running within a single region, AWS writes the fee to the billing file by applying a Surcharge to one of the running Instances.
If we have multiple Dedicated Instances running within a region there is no option within the AWS platform to designate which Instance is charged this surcharge.
As CloudHealth presents the data in the same format as the AWS billing file the surcharge is not split evenly between all dedicated instances.
This is important to consider if you are categorizing your report by a Perspective as the Dedicated Tenancy costs will follow the Instance that AWS writes the fee against in the billing file -