because some of your posts got deleted so your activity goes down
I got no messages of deleted posts, however I did delete like 5 old 1 liner posts myself so I guess that must be it then?
Because of my number of posts I thought that wouldn't matter.
Number of posts are indirectly always bind with your activity.
I guess, you had deleted all the posts which were belonging to same activity period. You need to check your post's dates if you plan for getting rid of your old spamy posts.
One quick help here :
www.bctalkaccountpricer.info could be used to list out your posts in activity period wise. So, you find any activity period with only one post and if you happen to delete that then your activity will decrease.