this bounties are mostly being checked by bots, and bots understand the numbers only, they can't check if you had made any post anywhere, so they just check number of posts and does the math......
Each forum post has its own post id. It is a combination of topic id and msg id.
I think the bot can record the last post id of the past week, and then count the new posts with id newer than the last one.
Although the current method is more simple, it is inaccurate and should be improved.