The ELI5 way to understand the magic perception thing is by comparing it to sound, people can only hear certain frequencies and everything over or under that threshold is imperceptible.
The ELI15 is comparing magic to electromagnetic radiation or as is better known "light", people only can see a small range of the spectrum and the frequency of the photons is defined by its energy from radio to infrared and from ultraviolet and upper frequencies are invisible for humans, so the MC's magic aura is imperceptible to other because the energy is so high.