The reason this happens is because the CloudHealth datasets for the monthly and daily time intervals are built differently.
In some cases, you may see multiple entries or line items within the Azure Statement Detail view in CloudHealth for the same service type on the same day in the daily time interval. The cost values for those entries may be different.
An example service might be for Sql Elastic Pool.
This may occur even if there were no size changes made on that day. The way the data is processed is the following: