See comment# 7. Drug companies used thimerosal as a preservative in a number of common childhood vaccines that used to be distributed as multiple dose vials of each vaccine, e.g. 100 doses of MMR or DPT or inactivated polio, etc. MMR means the combined measles, mumps and rubella vaccine, and DPT means the combined diptheria, pertussis and tetanus vaccine. Now these combined vaccines are only administered as one single dose from a single vial for each kid.
One speculation was that in those 100 dose vials the mercury would tend to settle at the bottom. So one person getting an injection early on in the vial’s use would get hardly any exposure. While the last child could get a mega dose.
Another issue is that mercury toxicity depends on what other medications are present in the body, and what other minerals are present in the vaccine and or concentrated in the body. For example Tylenol apparently dramatically increases the toxicity.