The stock airbox takes in air that's >15*F warmer than an open filter. So, to maintain any given cruising speed, the throttle plates will be open more which reduces pumping losses. Also, slightly more fuel gets vaporized due to the extra heat so the fuel burns more completely in the combustion chamber.

At high altitudes, the rarefied air also requires a wider (less restrictive) throttle opening to maintain the same lower altitude speed. Air resistance against the bike and rider is also less and a lower octane can be used because dynamic cylinder compression goes down. It's said there's about a 10% increase in fuel efficiency at 10,000'.

Okay, so Joko behaves on the bike & Pete is a hooligan.  That's where I'm betting most of the mileage variance comes from.  I did monitor the mileage before the Koso...didn't notice any changes.  I run 35F, 37R tire pressures & weigh in at around 210 with gear.  I get about 105 miles before the light comes on, with a few instances slightly higher/lower.  I got Ivans flash after putting on the K&N cleaner (not the round one) & the Danmoto slipon.  I think the plugs show slightly richer but that could be just in my head.  The only reason I questioned  this is because the first trip I made, I didn't think anything when the fuel light came on because I was about 18 miles out & thought there was plenty of reserve to get me home...then the dreaded sensation of running out of fuel happened.  I keep the clock function on the Koso.  When I cycled through to the trip meter it showed reserve came on at 90 miles.  I would have known something was up if I saw that mileage.  It's all good.  LIke you said Pete, gas is cheap at the moment.  Still wondering about the Stryker pulley.  Can't find any posts about gas mileage increase with the swap.