That's definitely interesting if you are seeing increased block production when targeting length 10 instead of 9.
I'm aware that the distribution of the fractional remainder isn't really uniform. That's one major issue which may be screwing up my calculations. I'll need to check how big the impact is at some point. I'll also need to double check my earlier calculations too. I have already accounted for the fact that lots of 9-chains are found when targeting length 10. In fact, I assumed that ten times more 9-chains are found than 10-chains when targeting length 10.
Also, if you look at the difficulty charts from a historical perspective, once a target length is breeched the network difficulty quickly increases to the x.3-x.4 range - and I expect this to be the case with 10 as well..
I would argue that the difficulty increase to the 9.3 range was caused by the momentum left after the transition to 9.0. The block rate was about 1.5 blocks/minute on July 22 after the transition to 9.0. Cutting that rate by 1/3 brings it to the target rate of 1 block/min. The difficulty rising to about 9.3 does just that. The chart also shows that the block rate was about 3.0 blocks/minute before the transition. That means that going from 8.996 to 9.0 somehow seems to have cut the block rate in half.
This one is an open question for me.. but I think it might be working against us. I see an abnormal number of 9.99999 chains and never see the corresponding 9.000001 chains. But the possible reasons behind that are over my head, other than it is non-linear.
I looked at the fractional remainder a while back. I remember seeing a large number of 0xFFFFFF (this is the 24-bit version of the fractional difficulty) values which should statistically be nearly impossible (if you wanted to assume that it would be uniformly distributed).