The reason why the top score in one year, can be lower than in the previous year, is that the test (the 100 structures to guess) is always new and different, so it can end up being 'harder' than the year before. Luck will also play a small role.
Another explanation for a reduction in the top score would be, that previous winners are not re-submitted unchanged. For instance AlphaFold v1 seems to not have been submitted to the latest competition.