If you export failed entries from posting, then repost to the failed ones, why do more succeed the 2nd time around?

There are many reason for this behavior.
- The site could have been off line or overloaded the first time around
- The connection could have timed out
- The proxy used the first time could have been bad

All this could mean that a 2nd. run could post some additional comments.  There are too many variables to try and control every time.  Best practice to maximize a run, is to lower connection and increase timeouts.  If you need maximum links from your runs, plan on reposing to the failed.