A lot of schools were sitting on decades-old Outstanding ratings, earned when the framework used for inspections was very different. Now that such schools are no longer exempt from routine inspections, it is no surprise that they score lower on the new framework.
A significant weakness of the old framework was the weight it put on raw results, meaning that selective schools found it relatively easy to reach Outstanding compared with schools with more varied intakes. The new framework has its own weaknesses, and does seem to be being used politically (eg to disproportionately downgrade non-academy schools to drive the Government’s agenda), but there is a stated intent to confine ‘outstanding’ to a much smaller number of schools, so this type of result is to be expected.