It depends what posts you deleted.
If you deleted posts which were necessary for your activity (posts which were within the 14 posts in a 2-week timeframe), that's obviously going to dent your activity. Activity =! posts.
You don't need to desperately scramble for high activity anyway.
This is an interesting case. I wonder since it depends on the time you are in the forum does that mean once you deleted the post that was counted in the activity would really cross out 1 post (1 day) in your activity count time frame. I guess this is a bug. He says he deleted 20 post (and assume that all of it is within activity count) then he should have 148 activity remaining.
You better contact admins so that they can fix your case and the bug so it will not happen again.