This activity system has always been the activity system, replacing a raw post count system. There has been no rule change since its implementation.
That simply cannot be true.
I went on vacation last Christmas at the Sr. Level with activity of nearly 1,500 and post count of nearly 1,500 and I came back after about 3 weeks with the same post count but my activity dropped massively to a lousy Jr. member level. I was days away from becoming HERO and I had to start over but this time adding activity points become way more difficult.
So clearly, the activity algo was changed in a big way; I know this for a fact cause I had to start over 8 months ago and I'm still not where I was before I made the mistake of taking a little time off.