Those predictions were before album release, like the original 750k prediction for 1989.
This prediction is based on actual first day sales.
Not entirely; they won't have full, accurate first day sales reports (which even then are only estimates) until tomorrow, at which point a new prediction will be made (one which was like 900k for Red but which was an increase from earlier). Tomorrow we could see them say 900k-1m or 1m+ if the final first day estimates are good enough!