An interesting phenomen is occuring with the Buell Thunderbolt; it will operate with much leaner mixtures than a 1200 Sportster while maintaining good drivability, and tests much better for exhaust emissions. The only difference between the engines in the two bikes are much less restrictive exhaust and intake systems on the Thunderbolt.
I suspect that the Sportster, with a fair amount of built-in exhaust gas recirculation because of its plug-like mufflers, requires a rich mixture for a decent flame speed; effectively, it's operating at a leaner mixture than you would expect just by looking at the intake charge. The Thunderbolt doesn't do that, so its lean limit is essentially extended downward.
I don't know if the following is true, but I'd expect the Thunderbolt to have lower cylinder head temperatures, to like less spark lead, and to have higher NOx emissions -- which fortunately aren't a part of the motorcycle emission standards.