I'm going with the idea that all three companies put out lots that had different strength levels.
This was to figure out the problem of rapidly waning antibody levels that they obviously knew about.
They used Vaers feedback to gauge which batch was causing problems so they could adjust production and continue onwards.
The masses became the experiment, in fact nobody is hiding the fact we and our kids are the experiment.
Any other explanation would probably be so complex that occams razor would bring us back to this.