Please be sure you're using IMAP on the tablet. I know you said you do, but it's worth checking

Long press the account, choose Account setup, then Manual.. Account type, IMAP or POP3, is listed near the top of the next screen.
Second, please open Options and Folders / Account options for this account, and check that "Remove deleted messages" is checked / enabled. This will make the app remove from its own storage messages deleted from the server.
Finally, if you get the unread mismatch again, please long press the account on the tablet, choose last item (Diag: message dates), then choose the Inbox folder and run the test. At the end, you'll be prompted to send me the resulting log file. This log has nothing personal in it, but it does have message flags as reported by the server, so I can check if the server sends read/unread flags properly.