My theory on higher rewards on long blocks is that the longer the block the more likely some people will disconnect either to hop to another pool.. or any number of other reasons.
The bulk of their reward ends up getting distributed to the rest of the pool because they did not stay connected the entire time.
I actually am collecting slightly more reward on long blocks than short. My assumption (possibly inaccurate), is that I'm in the top 50% for hashrate so the reward eeks up as time elapses.