Sometimes you might notice cost fluctuations on cost reports.
These changes, when not related to Cloudyn recalculation, might be related to new files provided by the provider out of the usual daily or monthly reporting schedule.
As you know, during the month all the billing files are an estimation of your daily costs and frequently (this is more frequent with AWS than Azure) updates to this data is done, sometimes even on the same day.
Usually, when the billing calculation for the previous month is finalized (usually by the 10th of the month), and the final billing file is provided, the numbers you see will stay stable. However, don't be surprised if you do see changes, since it's not unusual to receive adjustments from the provider (credits, etc.) even months after the relevant month was "closed" and these will be reflected if a recalculation is done.
This can also affect the numbers provided daily for the current month, as we always look back a few days into historical data, to make sure all adjustments are taken care of and that the application is displaying numbers correctly.
To sum up, the numbers displayed at the end of a day or at the end of a month should be stable but in some scenarios, some fluctuations can appear due to new data made available by the providers.