By Outlook do you mean the webmail, or do you mean Outlook as in the software program in microsoft office (I know micorosfot has poor naming systems sometimes)?
If you are talking about the software in office then you likley have your email setup as a POP3 email. POP3 is a very old protocol desgined in the era where you had the ONE family computer that everyone used. Now people have at least 3 devices they wish to recieve the same email on. POP3 works locally, that means it delivers the email to the computer and then wipes it from the email server, thus when your other computer connects to the email server to sync messages, that email is gone.
In your account settings you can tell outlook to leave the message on the server for X amount of days and delete on server if you delete on that PC. That will sort of help keep it more in sync.
The proper way to have your email in sync though is to use IMAP or Exchange email which keeps all the email on the server and your PC just syncs to it.