That's not what it means. This is a style of writing common in some academic fields where when they measure multiple qualities, they'll present them in the abstract next to each other in parentheses rather than repeat full sentences.
You should read "All else being equal, those highest on cognitive ability experience a 22% (53.2%) increase in the probability of realism (pessimism)" as as a 22% increase in realism and a 53.2% increase in pessimism
You should read "All else being equal, those highest on cognitive ability experience a 22% (53.2%) increase in the probability of realism (pessimism)" as as a 22% increase in realism and a 53.2% increase in pessimism