How to caculate my whitespace?

Hello Everyone,

What I found out in Exchange 2010 SP3 is, let's say, there are 2 mailboxes placed on the Mailbox
Databse TEST2 and the total size for each of them is 41MB and 42 MB, in total
it is 83MB around.

By running this command: Get-MailboxDatabase -identity TEST2 -Status | Select-Object Server,Name,AvailableNewMailboxSpace,DatabaseSize

Total DB Size is 264MB and white space | AvailableNewMailboxSpace is 31.56MB, 264-83-31=150MB, and where is the left 150MB space?

Thanks!

KeJiaLi