Not wierd at all though...
It is Rogers Yahoo mail which doesn't stick to the proper POP3 protocol.
The POP3 protocol says that whenever you read an email it should be flagged as 'read' and not be included in the final returned unread mail number. It is the server itself which returns the number of new (unread) emails. If it doesn't update that whenever you read a mail in your inbox (and leave it there) it doesn't stick to the POP3 protocol. There is nothing Plus! can do about it. It is a fault of your mailserver.
This is the same issue as with Google:
MsgPlus! Pop3 Gmail Limitations
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