the grade boundaries relate to the difficulty of the paper - i did my gcses this year, i'd say the 2023 exam papers were slightly easier than previous past papers overall, explaining the increase in grade boundaries. but throughout the year when i was revising i went by 2019 grade boundaries and ended up with all 8s and 9s so i'd say that they are a pretty good indicator of where you are, so don't worry, just using 2019 grade boundaries as your guide. at the end of the day grade boundaries change from year to year and it's not like you can predict 2024's so don't worry about them