This is most likely due to timing of when the S3 bucket was last synced to the platform.

S3 metrics such as object count, storage in GB etc are derived/parsed from the same job that gives you the S3 usage reports.  The S3 usage report is an extra day behind the S3 cost report (24hrs behind).  This lag occurs because S3 usage is calculated as a metric and CloudHealth metrics are not processed until a complete day’s worth of data is available.  Because cost and usage processing currently runs at 8PM EST in the CloudHealth platform, the complete data for the day is not yet available.