There may be changes to the underlying data that a perspective is utilizing that will change the data at different points throughout a given month. For instance, if a report is pulled on 1st and 10th of every month the data may change due to potentially different has different costs and groups associated to the perspective on the 1st vs the cost and groups on 10th.
AWS, Azure, GCP
If a perspective is based on tag and if there are any changes in the assets (more or fewer items are referenced, for instance) containing that tag, the data will change accordingly