There are a few reasons that GCP costs may not line up as you would expect. This documentation goes through 3 common scenarios:
1) For GCP costs, CloudHealth data is based on the BigQuery Billing Export. A BigQuery Billing Export will only include data from the date that the export was created.
Be aware that your BigQuery dataset only reflects Google Cloud billing data incurred from the date you set up Cloud Billing export, and after. That is, Google Cloud billing data is not added retroactively, so you won't see Cloud Billing data from before you enable export.
Additional details can be found here:
https://cloud.google.com/billing/docs/how-to/export-data-bigquery
2) Cost History will not match History Invoice for GCP due to the data source.
Data will be different between the two reports:
3) The platform assigns Enterprise Support charges (and potentially other costs) to specific projects, while GCP leaves them as generic costs not associated to any project.
This article goes into detail regarding how these costs are represented differently in the platform vs the GCP Billing Console:
Note: As of August 11th, 2022, customers have the option to disable this automatic distribution of project-level GCP costs on a go-forward basis.