The U grades (and other large drops) appear to be the result of a rounding issue when assigning ranked students to their grades. It'll be hard to explain via tweet but I'll try.
Please bear with me as I use a simplified example.
Now, coming up is the magic I will never know, because I don't have all the data, but OfQual have described it quite comprehensively.
They decided it wasn't, adjustments needed to be made.
A* 7%; A: 15%; B: 22%; C: 52%; D: 3%; E: 0.5%; U: 0.5%
I made these numbers up but let's pretend I ran them through "the algorithm".
This is what my 20 students have to fit into.
Notice I haven't told you their grades. It's now irrelevant. The rank is king.
Student number two would take us to 10% but wait I'm only allowed 7% so A grade for student 2 (Their CAG was A* - drat).
Students 3 and 4 take me to 20% so they get As (I'm allowed 22% to get A*/A)
I've got 55% worth of students (11) left but unfortunately only space for 10 in my Cs.
So I've one student left. We've used 96% of our grade allowance.
Surely they'll get a D.
Nope, only 99% of my students can have a A*/D.
Fraid not that takes us to 99.5% not not a full 20 students worth.
But a U. I've got 100% now. So my student can go here.
And that is how rounding down gave C students U grades.
The word rounding I've used adds a little confusion as in reality the student will hit the bucket for the nearest whole student. My numbers in "kicker" tweet didn't reflect this so 20: E not U.
Like OfQual I should have checked my work here.
Still can't be 100% sure, but I'm pretty convinced I was right first time and that in general the cumulative groups were rounded down so bottom ranked students got U as a matter of course if the curve had space for them. Sadly.