How is it only a Senior Member if it's from 2011, you only needed to post 100 more times to be a Hero. What a waste.
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)
The "time" value in the activity formula depends on the number of period you have
posted, and so an old account doesn't necessarily have a high "time" value, especially when the word "inactivity" is mentioned in OP.