I was not sure..
So let's take 23rd of Dec.
If someone will open an account 22nd of Dec and till 24th of Dec will post 28 posts (22.12 - 14 posts + 24.12 - 14 posts) - he should get 28 activity points in 3 days or even less.. Thats just my thoery

You're right, he would get 28 activity points in only few days (or even few hours) because he posted in 2 different periods.
To be more precise, it's because he posted at least one message in each of the 2 different periods (28 activity would be also reached with just 1 post on 22.12 and 27 posts on 24.12).
Activity is related to status, this is an unusual example of these two heroes agreeing about something
The next logical step would be to cut the status of people who haven't posted in X time interval.
You can't be a Hero if you've not posted the past week, you can't be Senior if you've not posted the past month. It'd help with the buying/hacking old accounts to prop scams.
This.
Hmm interesting.. But this can happen, or this is just talking?
It was just talking, at least until now.