I will need to check the code but I suspect it's just the number of posts between the two dates.
The post count not being incremented for certain forums is fairly simple to achieve as it is incremented, or not, when the post is made.
I'm not sure it's possible to factor that in to this easily as the post count per forum can be enabled/disabled at any time, with no tracking, so it's not possible to know if any posts made should or should not be counted.