“The threshold I.Q. level for a diagnosis of mental retardation has been progressively lowered over the years, in part because of awareness of the damaging social prejudice suffered by those labeled “retarded.” In 1959, the American Association on Mental Deficiency set 85 as the I.Q. below which a person was considered to be retarded.In 1992, the renamed American Association on Mental Retardation lowered the mental retardation “ceiling” to an I.Q. of 70-75,but many mental health specialists argue that people with I.Q.s of up to 80 may also have mental retardation. Flexibility in the I.Q. standard is important because tests given at different times may show slight variations due to differences in the tests and because of testing error — the standard error measurement on I.Q. tests is generally three to five points.”
https://www.hrw.org/reports/2001/ustat/ustat0301-01.htm
In the context of what was known during my college years, the IQ test had a +/-5% repeatability over the lifetime of an individual.
In the current time, there is some evidence that "neural plasticity" can be trained and improved. The BrainHQ program has demonstrated success.
Working on the assumption that the ability to perform a given job has a minimum IQ floor, the higher that threshold, the fewer candidates you will have to fill the position. When you overlay the IQ spread of populations, it explains the number persons from that group that are capable of meeting the minimum. Just because a certain group represents X% of the general population is not a valid reason why that group has the same X% of candidates meeting the IQ minimum to do that job. That might be achieved if the pool of candidates is much greater than the available number of position i.e. 20 positions and 5,000 applicants.