On GCSE grades and A level predictions, DS1 didn't have perfect GCSE grades. For the hardest to get into and high in league tables A level sixth form college, he needed an average of 7.5 across his best 8 GCSEs (out of 9 in his case). He averaged 8.25 across his best 8, 8 across all 9 (4 9s, 2 8s, 2 7s and a 6.
On A level predictions, I am not sure what DS1's predictions would be outside of the prestige university application situation as it is now. He doesn't think he'll get an A in Philosophy, he doesn't assume he will in the others. He does already have an A in Maths and is now studying Further Maths this year, and his other subjects over 2 years. His offers seem still seem quite high to me, but quite achievable for him, particularly as they're based on an achieved result and two out of three grades next summer.
However, if the expectation is that students need predictions of 3/4 4 A*s to get an interview, I think it's most likely that schools will make that most optimistic prediction to give students they may have encouraged to apply to Oxbridge etc, who have progressed well in sixth form and can see that they could do well on their chosen courses, the best possible chance. It's not just the students who are in competition here, the schools are too.