Not only amplitude but the bandwidth/q is significant. Psychoacoustically, boosted sub/lower bass will mask other bass and mid frequencies less than an equivalent boost let's say beyond 120-150hz would mask elsewhere. Boosts past this point are almost universally considered bad or deleterious to accurate timbre, usually result in "muddy" sound is the term that's always thrown around, and in contrast you can get away with much more in lower frequencies lower than that before things like intelligibility become a major problem. This will obviously present itself more obviously in some genres, for example in a tune with vocals it will be more obvious to almost everyone.
The fact that harman iem target requires more gain in bass than their oe target leads me to suppose that the lack of low frequencies interacting with our body/pinna/room (which still doesn't explain the quantity of gain- bone conduction?) or perhaps the psychoacoustic effect of partial/full occlusion has an effect on how these frequencies are processed. I would like to assume that completely flat has to be "objectively" wrong as far as timbre accuracy in lieu of reflections, and maybe even as far as general preference is concerned for sounds with incidence close to the ear/different angle than is expected normally. As far as I know variance in low frequency transfer functions between individuals is quite low relative to other bands, so individual physiology explains that even less like it does with higher frequencies. There could be a lot of mechanisms, maybe since internal/infrasound may be more perceptible with occluded ears, we need the higher gain to mask or interpret these frequencies better?