I haven’t seen the specific grade boundary you are talking about and am most familiar with CCEA grade boundaries, but the premise is the same across all exam boards.
Grade boundaries are set following the marking of all papers and are decided on a bell curve meaning that a certain percentage will achieve each grade.
In CCEA it is always 80% for an A for instance, and 90% for an Astar. However, those percentages are for the UMS - Uniform Mark Scheme - and the raw marks needed to convert into those UMS are changed every year.
Take for example the Physical Geography A2 paper - it is marked out of 70, but those marks are then translated into a UMS out of 120. (Total across all 3 papers is 300 - 120 for physical, 120 for human and 60 for DME)
In previous years, you have needed around 47/70 (approx 67%) in raw marks to convert to 96/120 (80%) get an A. That means that an Astar is likely to have been approx 59/70 (approx 84%).
This year, that paper was particularly tough and you only needed 43/70 (approx 61%) to get an A, meaning that you likely needed around 56/70 (approx 80%) to get an Astar.
The UMS grade boundaries stay the same for the duration of the spec, but what changes every year is the conversion from raw marks to UMS and this is dependent on how tough the paper was and how the cohort as a whole found it.