It changed about 3-4 years ago.
See this thread and my post 8 (and the link in post 8 for Matt's answers).
https://forums.ootpdevelopments.com/...=adjust+weaken
In your screenshot, the game will be creating an AB denominator based on year n + year n + year n-1 + year n+1. In 1970, for Yaz, the numbers are 603+566+566+508 so a total of 2,243 AB ... well above 600 AB.