pointythings
I can see it happening if the cohort size was such that the algorithm was used, but the cohort was differently distributed in different years.
Just as a crude approximation, let us suppose that you have a school in which, historically, you have had 45 students in a ratio of 2 As: 3 Bs: 5 Cs,:3 Ds :2 Es. The overall 'average ability' is the same this year, but instead, they are all Cs.
The school is asked to rank all pupils, which it does, and submits CAGs, all Cs [these are ignored, as the cohort is large enough for standardisation]. When the algorithm is applied, the lowest 6 students in the ranking will all be given Es, because in a typical year, that is what the lowest pupils get.
The same, with an extended range of grades, could also result in a U rather than a C - the 'lowest pupil' in the cohort will get the 'lowest mark of the usual annual average', regardless of their CAG.