People from the finance side use MM to mean million, which as you note is really 1,000 X 1,000. The other side you see it is in CPM which is cost per m (being

People from the finance side use MM to mean million, which as you note is really 1,000 X 1,000. The other side you see it is in CPM which is cost per m (being thousand). So people with this background tend to adopt the MM = million because that's how its done.