When using IMAP clients to access your email, the Inbox and any other user-created folders are synchronized as a reasonable person (my grandmother) might expect – you add/change/delete content on one device and the change is synchronized to the server and to the other devices.
However, IMAP appears to treat the drafts, junk and trash folders (I’m not meaning to specify the literal names of the folders as specific mail implementations like Gmail or Outlook might name them – so by “junk” it might be the “spam” folder and “trash” might be “Deleted items” and so on, depending on the client) differently. In particular, the drafts folder appears to be client-specific (no sync) and I’ve seen similar oddities with the junk and trash folders.
What is it about the IMAP protocol that results in this?
It depends on the client.
It could be that the client is storing messages in those folders locally. In particular, in order to support a “Sent Mail” on the server, you would need to send the message twice: once to the SMTP server for actual delivery and then again to the IMAP server to store it in the “Sent Mail” folder. That’s why many clients will save Sent Mail locally, rather than on the server.
I assume the same is true of “Deleted Items” or any of those other folders.