How are AS and A2 grades combined? (CIE)

From resources online, I read that they'll take your PUM from AS, add it to A2 and then take the average. However, my teachers told a different story. I took the Oct/Nov 2021 exam for AS level, and got pretty good marks due to the low thresholds. My teachers told us not to get too 'complacent' because they apparently dont take the PUM to calculate your A-Level grade but instead take your raw marks. They then said that they take the raw marks from your AS exam, add it with your raw marks from your A2 exam, and use the grade threshold from the A2 exam to calculate your grade, which honestly is really scary to me, because I'm not sure the May/June 2022 thresholds will be as low. It also doesn't really make sense to me? I thought the purpose of grade thresholds was to account for varying difficulties in each exam? Why would you use a different grade threshold for an AS exam? TL;DR wondering if they take the average PUM of AS and A2, or if they add up the raw marks and apply the most recent grade boundary