This is due to the way Azure reports VM usage in the billing data.

Azure reports the License cost separately under the "Virtual Machines Licenses" category for only non-Azure Hybrid Benefit VM which is covered under reservation.

Azure doesn't report the License cost separately for the non-Azure Hybrid Benefit VM which is 100% on OnDemand. 

CloudHealth, on the other hand, will take both the Compute and License costs and combined them for all VM types and present the cost under the "Virtual Machines" category.

Below shows example data regarding the categorization differences of "Virtual Machines Licenses" and "Virtual Machines".  If you check the non-Azure Hybrid Benefit Windows VM which has RI-covered usage from the ARM VM Hours report, you see the following:

OnDemand Hrs: 741.95
RI Hours: 2.00

OnDemand Hrs: 743.65
RI Hrs: 0​