Activity on this forum is not working differently for every users, it is the same. Every 14 days, 14 would be added to your activity. But if you are not posting continuously and your activities reach your posts number and you are not posting, next posting up to 28 posts may add up to 28 activities at once.
This is true, but I think the activities do count more than this if you have created your account for a long time and have not been able to post to meet up with the activity count. Let me use my account. For example, I created this account in July 2022, and I was not active on the forum.
I posted once in a while, until late 2022, before I later started being late 2023, and ever since then my activity did not stop adding till I was able to accumulate my full member activity requirement, which was the last time I took note of it before it stopped counting, and now it takes every 14 days to update.