Correct. Think of IMAP as a one way synchronisation with the mail server, where as pop is a straight download.

If you are downloading messages you can either leave a copy on the server or delete after download. With Synchronisation if you delete a mail locally, you need a method of showing that you have deleted locally but not from the server and therefore another method of deleting from the server as well. Some mail clients will do it in one hit... but then what happens if you have hit the wrong button?

It seems clumsy under IMAP when you are used to POP3, but it is better to do it in two stages in case of mistake.