Ahh, I'm with you. So the forum software recalculates activity on an hourly (or close to hourly) basis, rather than once every two weeks.
But it only seems to do it for new members. Any posts I make now, won't be credited until the end of the period.
Actually, I've just had a thought. Because I've got a load of potential activity, it may start the update at the beginning of the period. This means that as long as I've made one post, I'll get the full 14 at the beginning, and then have to wait for the next period to get some more. If I don't have 14 of potential activity, then it will top it up as I post until I get to the 14 limit. That would be esier to program that having to check ranks.