Russ, here is my guess on the dilution effect you are seeing where the gas with ambient air causes the piston to move further.
Ambient air is 79% is inert gases. So the mixture will slow the burn rate down on the H2 burn. The military teaches a few things about burn rates in explosive compounds. The slower burners are classified as movers while the fast burners are classified as crackers. This is a very simplistic look at explosive compounds but may be useful in determining your burn rate and movement issue.
I am guessing that because you diluted the gas down with inert it slowed the burn rate of the hydrogen down which caused it to expand in the pressure chamber over a longer period of time. While that is taking place the piston is moving forward and the pressure behind it starts to drop, but the gas is still expanding and exerting a force on the backside of the piston which will produce more workforce.
additionally, in one of Stan Meyer's videos he states that he recycles exhaust gases to control the rate of burn in the cylinder of the engine so its burn rate closely matches gasoline. This causes me to consider also some other guy on youtube that showed a civic running on HHO or browns gas. In that guys process he had to retard the ignition to TDC or just before TDC because the HHO burned so quick, under normal timing it would fire before the piston reached TDC. Need I say that is a very bad idea in an ICE?
Anyway I think you have found why the military is constantly looking at new complex chemical compounds with various burn rates. They design explosives for different purposes. I will give you an example they showed me in school. C4 placed under a tree trunk did nothing to move the trunk away from the ground. However, when they used TNT of the same size it moved the tree a very long way from its starting position. They were using this scenario to teach us the difference between a cracker and a mover explosive. C4 burns very fast when ignited and TNT burns much slower, so it builds up pressure over a longer period of time from the point of ignition.
Hope that helps. Keep up the great work.