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The authors show variance ratios of 1.11 to 1.21, I take a VR of 1.16. If we set the female variance to 1 this implies the standard deviation for female ability is 1 and for male ability 1.077. Using an online calculator for the Normal distribution you can find that given their standard deviation .0102% of males have ability of 4 or greater (4 female sds) but given their sd only .0032% of females can be expected to have the same level of ability, thus a gender ratio of 3.18.

Note that we are assuming that mathematical ability is normally distributed – we know the data fit this distribution around the mean but we don’t know much about what happens at the very top.