Probably not - it's a simple call to delete where you pass in a flag to tell it to get rid of the mailbox, too - it's not a seperate call or anything.
More likely you have Exchange setup to keep deleted mailboxes around for a while (this is default). to check this open the Exchange system manager, drill down on Server | storage groups | to the mailbox store in question - right click it and hit "properties". On the "Limits" tab you'll see an entry for "Keep deleted mailboxes for (days)" - it'll probably be set to 30.
