Exchange 2010: mailboxes+whitespace=database file size?

We have stand-alone Exchange 2010 Version: 14.00.0682.001 on Server 2008 R2 standard, physical machine.

About a week ago, I had to migrate all mailboxes from a previous database on this same machine to a new database because of corruption in one user's outbox that caused the .edb of the previous database to grow out of control (.edb was 463 GB with mailboxes totaling less than 50 GB). I moved each mailbox, choosing not to move corrupted items. Now, only the new database exists.

I've been monitoring the size of the .ebd file each day and comparing it to TotalItemsize + TotalDeletedItemSize + AvailableNewMailboxSpace(whitespace) and noticed the sum of these is not equal to the size of the .edb file. Is that the correct way to determine what the .edb file size should be, or am I missing something? The .edb file has been growing about 300 MB per day for the past few days, so I wonder if the database migration even solved the original problem.

Here's how I'm coming up with the numbers:

GENERATE .CSV OF SIZES OF EACH MAILBOX:
Get-MailboxStatistics -Server “mail02” | Sort -Property TotalItemsize | select DisplayName, LastLoggedOnUserAccount, ItemCount, TotalItemSize, TotalDeletedItemSize, LastLogonTime, LastLogoffTime  | Export-CSV c:\mailboxsizes.csv

GET WHITESPACE OF DATABASE:
Get-MailboxDatabase -Status | Select Servername, Name, AvailableNewMailboxSpace

In CSV, sum of TotalItemsize bytes of all mailboxes = 35,590,476,231 bytes
In CSV, sum of TotalDeletedItemSize bytes of all mailboxes = 15,704,138,000 bytes
AvailableNewMailboxSpace = 234,800,000 bytes
35,590,476,231 + 15,704,138,000 + 234,800,000 = 51,529,414,231 bytes
BUT…
.edb file size=55,709,335,552 bytes. What accounts for the difference?

I have default settings for database maintenance (run daily from 1AM to 5AM, background maintenance enabled, circular logging enabled). I run nightly full backups using Backup Exec 2010.