Some people retain mercury and do not eliminate it easily, so it simply goes in and pretty much doesn't readily come out, so poisons them continually.

Others eliminate what goes in pretty efficiently, so it usually shows up readily in their hair and urine levels, which shows they are ridding it into the excretory areas. Others can show very low levels in hair and urine, but be highly poisoned by it because it's not coming out.

Or the very low levels can mean low levels, but they are just easily poisoned and made ill by even low levels of mercury.

So yeah it's down to sensitivity/tolerance, ability to excrete and a person's natural constitution.

You can actually be exposed to mercury from the womb by your Mother's amalgams and be poisoned from the word go and then from the womb, to breast milk which contains high levels of mercury (if the Mother has amalgams).

It's very sad, but some people just do NOT deal with mercury on any level.