In the beginning there was no data, so we best guessed and experimented, until we reached where we are today. Part of reaching where we are today was the introduction of Gas Chromatographs into our "ghetto" processes, to see how we were doing, and the results were an eyeopener.
Ergo we found far higher residual solvents, than previously predicted based of human sensory thresholds for odor and taste.
In national concentrate competitions, not all of the samples failed, but surprisingly more than expected, even though each entrant was convinced enough that he had the winning concentrate, to plunk down high entry fees to enter.
Attached are the FDA standards for residual solvents by classification, but local standards are typically significantly more stringent.
What happened as a result of those failures, and the publicity it drew, is that there was a general migration toward vacuum technology, which makes it faster and easier to remove residual solvents.