Hi Megan: The "roughness" is not caused by "misfiring". It's caused by different amounts of power being produced by the individual engines - each cylinder - bolted to the same crankshaft.
When you plot the power output of a cylinder against mixture, you see that the curve is 'steeper' on the lean side of peak. That means that smaller changes in mixture on the lean side cause greater changes in power than the same change in mixture on the rich side. That's why trying to run LOP highlights the extent to which engines come 'out of the box' with poorly balanced fuel/air across cylinders. Running ROP 'hides' the imbalance. That's also why successful LOP operations usually entail testing and swapping injectors (on injected engines) to get all of the cylinders close to each other on the lean curve. For those who achieve the nirvana of perfect balance, there is no 'lean missfire'. As the mixture is leaned on the lean side, each cylinder produces progressively less - but the same - power output, until the mixture is so lean that it can't sustain combustion and all cylinders simply cease to produce power.
andrew: Please save your questions for the APS.