There are a few reasons this could be happening:
1. The Azure account is not fully configured in CloudHealth
Always check the status of the enrollments (if applicable) and service principals within the CloudHealth console to ensure they are "Healthy".
2. The timeframe of the statement is not matching CH reporting
Azure billing takes place on specific intervals based on when the account was first created. In other words, it's very likely that the billing information will not match up to the default calendar-month monthly reporting that CloudHealth defaults to. The best resolution is to view the billing data to the specific date range using the daily interval.
3. Azure is placing some cost outside of the billing statement timeframe
Assuming the above two items have been addressed, there are still situations where the overall totals may be off by a small amount. This generally boils down to a difference in how CloudHealth represents cost and how Azure generates bills. For instance, CloudHealth will ingest usage data, apply cost based on the current rate card, and then associate those costs to the appropriate timeframe based on the timestamp. Azure, on the other hand, has been know to place some charges near the end of the statement on the next month. There does not appear to be a clear pattern as to what charges that will be moved but CloudHealth is ultimately at the mercy of Azure in terms of what will ultimately end up on a bill. It's a unique challenge that CloudHealth faces with Azure billing and there is currently no resolution.