It's about 39%. The problem is that in a normal year, everyone comes out an exam with a rough sense of how they've done. If you come out feeling like you bombed it then seeing your B become a C is not going to be a surprise.
The issue is that "predicted grades" are not expectation values in a statistical sense, if they were then teachers would be pretty consistently doing a bad job of forecasting them.
A predicted grade is based on an assessment of ability: "What do I think little Johnny is capable of when he takes his exam in however months time". Now in theory, results can be both better and worse than that prediction. In practice, the fact that the scale is closed ended (and many more are near the top than the bottom) means that it is more common for the variability of exam performance to lead to slightly worse performance. In other words, more pupils underperform their ability based prediction on exam day than outperform it.
So there is nothing inherently wrong with reducing grades from the centre assessed grades, many newspaper headlines are treating this like the CAG is the "real" grade that is somehow being taken away.
If you look at this table: https://twitter.com/Samfr/status/1293979033458417668/photo/1 you can see that the overall grade distribution produced by the model for this year is similar to that for previous years. Not surprising since that is by design.
The problem here is that Ofqual is treating grade inflation like it's the worth possible thing that could happen and has essentially sacrificed individual fairness on the altar of aggregate consistency.