I've checked the code now and confirmed that 'past' courses are those which have been completed and 'current' courses are those which are enrolled and not yet complete.
As for why this would happen on a teacher's dashboard - as far as I am aware the block is generic for all users on the site. It does not distinguish between users who may have the 'teacher' role on some courses and those who may have the 'student' role on some courses.