These are standard scores that represent a position on a curve. To translate to a percent, you only need to know the middle point and the size of a standard deviation. These tend to be 100 and 15 respectively.
So, whether it is an achievement test, an IQ, FSIQ, GAI, a military placement score, or whatnot. With the same mean and standard deviation, it translates to the same percentage. It's part of the appeal of standard scores.
Now whether IQ is in truth a Guassian or Ln-Normal or other distribtuion is another topic.
