The headless pirate attacks using sprite layouts 8:F2, 8:F4, and 8:F8. If you look at 8:F6 through the headless pirate, it just looks like a glitched sprite, since the sword is not aligned properly. But then I noticed the hand moves ever so slightly between those three frames while the body stays the same. And the more I focused on that hand, the more the sword's position started to make sense when viewed out of the periphery of my vision.
Other characters in gaming which fight a la fencing style often have either a rapid thrust or a flailing attack. As seen in the first image, maybe 8:F6 was originally part of a flail animation.
Or maybe it was supposed to wiggle like a door stopper. All I had to do to make the second animation was change 1 bit to flip the sword sprite. What if the sprite artist's software wasn't coded properly and the animation didn't work right, so they just said, "screw it," and skipped frame 8:F6?