I have Outlook 2011 (latest version is 14.0.5), and there is a "Junk" folder. Do you check what is in that folder? As one would expect, I receive Junk EMails every day, and get rid of them right away. That of course will take care of the discrepancy in the ones you receive.
The other thing you should do (actually do it from time to time) is to rebuild Outlook's database. Here is a link that explains one way to do that:
The other way is to 1) quit all
applications running, 2) press the Option key when launching Outlook, and 3) you will get the second screen described in that link.
After you rebuild it, a copy of the old one is made in that same folder (I believe it is called "Main Identity - backup"). Before deleting that back up one, first quit that "rebuild" process, then launch Outlook like normal. I suspect it will work, but it is using the new, rebuilt database, which is what you want.
To get rid of that backup folder, navigate to your user folder, then your user folder name, then Documents, then Microsoft User Data, then Office 2011 Identities. Once you are in that Identities folder, you should see at least 3 files. One of them is the Main Identify folder (which you just rebuilt), one is the "Main Identity - Backup" folder, and the other is a file entitled "MeContact.plist". You can just drag the "Main Identity - backup" folder to the trash.