Why The FCC maximum permissible exposure (MPE) limits are based on how much RF energy the body actually absorbs, and absorption depends strongly on frequency. A standing adult body acts roughly like a resonant antenna near 35 to 70 MHz, so absorption peaks in the VHF range and the limits there are the most restrictive (about 0.2 mW/cm^2 for uncontrolled exposure in the 30 to 300 MHz band). Above and below that resonant region the body couples less efficiently, so higher power density is permitted.
Watch out The idea that lower frequency fields carry more energy is backwards: photon energy rises with frequency. And low frequency RF does penetrate the body, it just is not absorbed as efficiently as at VHF resonance.
Limits are tightest at VHF (30-300 MHz) because the body resonates there, like a half-wave antenna for you.