It could be the "moving column" effect of air. If you get all the air moving in the rather long stock air filter system, as each cylinder's intake valve shuts, the remaining air which is still moving, adds a bit of "boost" to the next cylinder as it's intake valve opens. Intake tuning is almost a whole science in itself. At max rpm I would guess three K&N's would provide the most hp and torque. At lower rpm's the stock set-up may provide more power in certain ranges. It would be interesting to see side by side dyno charts of the same engine with K&N's and with stock intake ran one after another for the best comparison.