This is probably due to a bug (?) comes up once in a while, if I recall correctly, your number of games is not updated in real time but only occasionally, so if your sixth game ended recently, the 'procedure' checking whether you already have six games still 'thinks' you have six.
(Please note that Open Invites and challenges also count as games even if they haven't started, so if you have e.g. five games and started an open invite or challenged someone, you can't have more games)