At the beginning of the month, AWS writes a line with the full committed amount for the Dedicated Hosts. Then throughout the month when the Dedicated Host Instances are run, the cost on the usage line is $0.
To show how much is used vs unused, CloudHealth shifts the cost from "Dedicated Hosts Unused Cost" and moves it into "Dedicated Host Instances". This shift is what is causing the "credit" but it is actually just a shift in where the cost is reported.