Manwell Pablo wrote:how does that even work surely a achievement is worth its base score at least.
It's called ratio decay and is a bit tricky to explain. In the GTASC they take a snapshot of your TA score at the start of the week. Your current score for the week is recorded as (Your current TA score - The start of the week TA score). This can throw up some anomalies.
Imagine you unlocked a rare achievement before the cutoff worth 100 points. After the cutoff a lot more people unlock that achievement, and it becomes less rare. It's now worth just 75 points. This gives you a weekly score of -25.
This is why it's best to avoid getting hard achievements on new games, as they will always be high scoring to start with and then drop as time goes on. It works the other way too though, if you've completed a game which becomes a Game With Gold for instance, loads more people will start the game, increasing those ratios and giving you a large boost.