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Believe it or not... a lot of it is also caused by the intake system resonance. Take a look at the picture I attach here -- the upper 2 patterns are with the upper airbox cover removed; the lower one is with the cover in place. The model on the dyno is a 2019 Duke 390, no mods, stock exhaust, stock controller.
If you also wanna see what is going on with the A/F ratios during these dyno runs, reference the following thread and scroll down.
www.ktmduke390forum.com
This is the precise reason I installed a DNA top filter in combination with my stock lower filter.
If you also wanna see what is going on with the A/F ratios during these dyno runs, reference the following thread and scroll down.

PT V4
Some exiting news from Race Dynamics soon: https://www.facebook.com/338593929645625/posts/pfbid0hXgXzAXz2reLnMuxqmjFFg5q3i9dKtDzSR7ZmpYHGaBfFhfPxveJnUS5Py6dsZtVl/
This is the precise reason I installed a DNA top filter in combination with my stock lower filter.