I disagree that "The vast vast majority of IEMs are flat here (200-1kHz region)", many in fact possess a declining slope to varying extents in the region
I mis-phrased that, they are either flat through the lower mids OR they have a slope down. What the Variations and Harman has is a slope UP.
Many have some flatness in the lower mids, after a bass shelf. They come down (at some point), and then are flat to 1kHz. Flatness here tends to sound "clean" and keeps the bass boost from bleeding into the mids:
It's also common that an IEM has a declining slope from the bass all the way to 1kHz, that is super common. Many IEMs do that.
What Harman does (and the Variations does) is reverses that, it's lower at 250Hz than 1kHz and then slopes
back up to 1kHz. That's not common, at least I have maybe 30 IEMs and the Variations is the only one I have that does this. But this is actually the Harman in-ear curve, it does have this specific dip in the mid-bass. Maybe clearer here, plotted against IEF neutral which is totally flat from 20-1kHz: