How come I still have 98 Activity with an Account which is over 1 year old and over 300 posts?
Because activity only increase if you post at least 1 on every 2 weeks.
Here's the quoted post of theymos to support what you said and is
more precise than the words you used:
The activity number is determined in this way:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
Activity is updated every hour.
This is also included in the links given above but I wish to repeat it here since it is the
specific thing you are looking for and putting some
emphasis to the necessary details in this case for which you may somehow overlook it. Hope this provides clarity in your part.