In the UI the "Items to Migrate" value is larger than the "All Items" under the Mailbox tab.
When calculating All Items, the value returned by Get-MailboxStatistics Items is used to determine the items to migrate.
During account discovery, mailbox statistics are collected. However, during mailbox migration, a separate assessment is performed to determine the actual items that need to be migrated. This can result in the following scenarios:
Estimated vs Processed
The estimated count is obtained from folder-level data at a specific point in time. The processed count is calculated after the migration completes, once all folders have been scanned and all messages have been accessed. The processed count is therefore more accurate than the estimated count.
Processed vs Migrated
Some items may have been migrated previously. These items are included in the processed count but are not included in the migrated count.
"All items" doesn't take into account the items within the Recoverable folder.
