The problem is there are really 3 parties here: Anglicans, Catholics, and the ecumenists. This agreement really only refers to the ecumenists, who are increasingly apart from the main thinking in their own denominations. The reality is that Anglicanism is steadily moving farther away, not only from the current Catholic Church, but from their own historic roots.
It is ridiculous to quantify doctrine, so the 80% is meaningless. The problem is that there are moves to eliminate ARCIC as an unneeded drain on staff. So ARCIC keeps putting out press releases to show progress, to maintain funding and staff time in the next year. What was the percentage in 1970, when Anglicans and Catholics were much closer together? 99%? Do you think if their stats showed their percentage was falling, they would put the falling stats in a press release.
Follow the money.