My school has noticed that other schools are very optimistic, so does the opposite - it is overly harsh with the predicted grades. A* are too rare, even if you are definitely capable of receiving them. My Spanish teacher says she never gives A* as a predicted grade because exams are so hard. All subjects, especially Biology in my case, have been very conservative with their predicted grades.
I presume this is partly so that everyone hits/exceeds their predicteds and meets the (lower) conditional offers, so most people get into their universities, albeit to worse unis that offer low grades or unconditionals.
I got an A in Biology in AS, yet my teachers put a B in my predicted for A2. It meant I couldn't choose a university like UCL for my course. After some emails, though, it seemed that they reviewed their decision and put an A, thankfully.
In some ways, this motivated me to do more work (and I did do more) for all my subjects that had been harsher in their predicteds.