Harness racing post position bias

[fun160]

I've begun studying the ponies and have noticed an odd disparity regarding harness racing post position bias.

On some tracks, such as Saratoga, there is a straight line for the post position statistics--the farther inside you are, the better the win percentage. At many (most?) tracks, the 5th post position seems to have the advantage, often better than the 1 position.

What accounts for this disparity? Are there two (or more) different starting procedures?