I personally, seem to get many postings deleted as I am about to take over the top spot in the SCN rankings. Interestingly enough, the postings that get deleted are often over 1 year old at that time. Come ON - these postings have been around for over 365 days and now they are now just being targeted because I'm getting more points than anyone else for answering relevant questions and posting newsworthy items about PB.
The point values shown in the leaderboard are a 12 month total, and only for that particular section. That's why vary considerably from your overall point score in SCN. If the posts that were deleted were over a year old, they would have no impact on your ranking in the leaderboard.
You may see decreases in your point rating in the leaderboard, but it would be because some points you had accumulated had scrolled out of the 12 month window they are using for the calculation.