Shoutbox

Plus! uses the POP3 protocol as defined by the official RFCs to read the amount of emails in your account.

However, if your email provider does not follow the official RFCs guidelines to the letter, it may result in the wrong number of unread emails to be returned to Plus!. Thus, if this is the case, Plus! can't do anything about it.

And GMail is known to not exactly follow the POP3 guidelines. The support is better than it used to be, but it still does not completely follow it properly (especially when it comes down to returning the amount of unread email).

Another reason can be that the returned number of unread emails are/were cached (some email providers do this), and/or that the time between checks was too small.

This said, it may also be possible that there is a bug in the Plus! email checker that it does not inform you of unread emails if that number is lower than the last check and the time between checks was very short (I need to test this to confirm it though - it is just a wild guess).