Counter for new messages in thunderbird don't set himself back to '0'

If you recive two or more new messages with Thunderbird at the same time (if you start up your computer, for example) there will be a small number next to the icon of thunderbird in the unity bar at the left site of your display. If you open only one of this messages, the counter don't count down, it says that you've recieved still two new messages altough there is only one by time. sometimes that is a little bit irritating.
If you've read the second mail, too, the counter removes itself. But who reads all his new mails? Who do not ignore some not important, but also new ones?