activity is changing based on an post number in last 14 days, by 14 if you have more than 14 posts, and by number of posts, if you had less, so that is why your did not changed
you probably missed to see that prior to reaching member status, but that makes sense to have more time needed to reach member status, or all above statuses, not just by getting some merits, and posting daily, you need time to mature your status as well
from my point of view, it is a good thing, and helps against spam
AFAIK, Apparently you can have the same post count and activity count without really waiting for 14 days to gain 14 counts of activities, just simply by having at least 1 hour or more interval with your post since you create your account. Once you spammed and posted within 1hr, there's a huge chance that the mods might skip the other post thus counting only on the most recent one. I'm telling this by experience and not really by the main thread with regards to activities, and I guess this still applies tho.