Activity works by taking the number of posts you have made within a 2-week period (capping at 14) and adding it to your current activity, which starts at 0. These values are stored in a database and updated with a script that runs periodically every other Wednesday.
This is not accurate. Activity is updated every hour.
Activity is updated every hour.
If your post count is smaller than 14 * number of two-week activity periods in which you have been active, you will see that your actvity will increase after every post you make.
If your actvity doesn't increase after every post you make, that's because your post count is equal or bigger than 14 * number of two-week activity periods in which you have been active and you have to wait until a new two-week actvity period starts and you make a new post in the new period.