I was trying to get the virtual mass extremely low. I was throwing every trick I knew of to maximize efficiency with a 3ggp arrow, but fell short of my goal. This particular design has stiffened lever-like limbs with a smaller bending area centered near the handle fades.
It makes sense why this happens. There are a couple natural frequencies in the bow limb and combination of the bow limb and properties of the string which either fall in or out of phase with the launch of the arrow. The big dip is driven by the natural frequency of the middle of the bow limb between the fades and string. This frequency is lower for a long-limbed longbow than it is for a shorter limbed static recurve, and this shifts the bigger dip in the plot so that the lowest virtual mass coincides with heavier arrows. It isn’t easy to change this very much without compromising other desirable properties of the bow such as smoothness of the bow or energy storage versus draw.
Alan