I have seen curves like that, and cannot really explaint why not all engines do this, I also have seen many engines that run perfectly fine with that kind os dipped curve, I have a few of those myself too...
What I do know is that there is, for a fourstroken an interference area between the valve's cycle frequency and the RPM. A cure might be to simply move the LS needles a bit to nudge the curve up or down. No guarantee.