The ratio refers to the share of natural scientists compared to the ethnic group's share of the total population. You can see that Americans of English, Irish, Italian and especially Scottish descent are natural scientists in disproportionate numbers.
The picture changes for mathematical and computer scientists. Americans of Polish and especially Asian Indian descent are more likely than other groups to have this kind of job.
The only two large American groups that did not each produce at least 5 percent of natural, mathematical, or computer scientists were blacks and Mexican Americans. (I was surprised that Jews were only 1.0 percent of the natural scientists and 2.9 percent of the math/computer scientists--small sample size does not help).