Your "Higher Female" filter is causing the viz to show the averages for occupations only where males get paid more than females. For those jobs, your calculation is accurate at 37.1%. However, if you want the gender gap calculated on all occupations, not just the ones where men get paid more, you can remove this filter and remove the "Higher Female" field from your rows shelf. Your "Wage Gap" field will recalculate to 33.9%.
Let me know if you wanted to calculate this in a different way.