In the scan details you can see the value of the bytes scanned. This value is differen than the value you can see in Outlook, opening your account details and checking the "folder size" value.
The folder size you can see from Outlook is always rounded off to the next 1024 bytes value. For example, if the bytes are 1469 the value will be 2KB (2048 bytes).
The scan value you can see in the DLP scan details is the real one of the data have been scanned. For example 1469 bytes.
Here other examples:
Scan of 2 emails - Folder size in Outlook 4KB and DLP scan details 2766 bytes
Scan of 3 emails - Folder size in Outlook 6KB and DLP scan details 4063 bytes
So the different visualization you have between Outlook Folder size and DLP scan details is due to the rounded off Outlook apply to all data. The DLP scan details is telling you the right amount of data scanned because it is really scan byte by byte.
And also I can tell you that smaller emails you scan, bigger the difference will be between Outlook Folder Size and DLP scan details snapshot.
Here some examples:
Scan of 3 emails with attachment, the different is very small: Folder size in Outlook 21463KB and DLP scan details 20941695 bytes (~2% different)
Scan of 10 emails, the different is bigger: Folder size in Outlook 16KB and DLP scan details 13016 bytes (~22% different)