I would say that, although the fine gradings between Good and Outstanding, or even Requires Improvement and Good, are not necessarily reliable / reproducible, there IS a difference between a school at the top of the gradings and one that is graded Inadequate.
By which I mean, while the same school inspected on one day by one team might get Outstanding, and on another day with another team get a Good, there IS a genuine difference between a Good school and one that is rated Inadequate, and that would be picked up regardless of the team and the day.
Was the school previously Good? Has it been some time since the last inspection? IME, what has usually happened in such a case is that although the 'headline' results of the school may be perfectly good, one or more subgroups within the school has not been making good enough progress , or getting good enough results, over more than a single year. This happens particularly if the school has been rather complacent about results, perhaps because they have a generally 'good' intake, and have not focused on e.g. the performance of the relatively small number of low ability or pupil premium children.
It can also happen if there has been a 'notable failure' in a single area, particularly those which are limiting judgements such as safeguarding (there are some areas which, if graded Inadequate, bring the overall level down to Inadequate).
Occasionally it happens if there has been significant staff turnover leading to a lack of overall direction and / or gaps in important processes and procedures.
The schools that get out of this situation quickest and most successfully are those that are open, honest and reflective about what led to the Inadequate judgement, quick to involve and communicate with parents and other stakeholders in terms of creating an action plan, and decisive in terms of e.g. appointing new leadership, training specific staff, bringing in expertise in particular areas of weakness. A blanket letter of refutation / denying the issue isn't a great start, tbh - I have seen successful turnarounds which start with a 'we do not recognise the school in this description but we are nevertheless going to do x, y and z to address the issues identified', but tbh the ones that start off with 'the inspectors were wrong because' do tend to take longer to resolve....
I have seen a (primary) school go from Inadequate to Otstanding in a year, and many from Inadequate to Good within 2 years. Secondary schools, partly because of their size, tend to take longer to turn around. In the past, Local Authorities have tended to pour resources into such schools, brokered partnerships with successful schools etc to aid a turnaround - if the school is an Academy, reliant on its own funding to e.g. bring in experts and advisors, I am not quite sure what happens.