how someone can have 287 activity? it should be either 280 or 294 or so on
You can easily have an activity score which is not a multiple of 14. Recall that activity is based on a series of 14 days periods in which you have posted at least once then:
max(post-count, active-periods*14)
For people who have post count > than active-periods*14, their activity is always a multiple of 14. But for people who's post count < active-periods*14, their activity is their post count, which obviously doesn't have to be a multiple of 14.
I hope this helps.