Historical Data Values for MS Usage (Email, Yammer, Teams) get Updated in Usage Reports on Office 365 Admin Page

Question

We noticed that historical values also change in the usage report extracted from the Office 365 Admin Page.
https://admin.microsoft.com/Adminportal/Home?source=applauncher#/reportsUsage

For example, when we extracted Email Activity report on 4/1/2021 (i.e., Report Refresh Date), the values for Report Date 3/31/2021 were as follows:

Email Send Activity: 96964
Email Receive Activity: 646489
Email Read Activity: 628318

Whereas same Email Activity report extracted on 4/13/2021 (i.e., Report Refresh Date), the values for Report Date 3/31/2021 were as follows:

Email Send Activity: 96972
Email Receive Activity: 646503
Email Read Activity: 628348

We have noticed such minor changes in other Usages reports also (Yammer and Teams). Could you please clarify why does the historical data gets updated for these usage reports.