Mathematics might seem full of stringent rules and calculations, but it's actually a very creative, complex process. Given the lefty tendency toward excellence in divergent thinking, it's not terribly surprising that they also tend to do well in math. This has long been the line of thinking, and a 2017 study published in the journal Frontiers in Psychology has added more evidence to the pile.
Researchers assessed handedness in more than 2,300 student participant students. Lefties (especially male adolescents) significantly outdid the others when presented with complex mathematical problem-solving tasks. However, handedness didn't make a difference at all when asked to solve basic math problems. Interestingly, participants who reported that they are extremely right-handed (as opposed to moderately right-handed) underperformed on all of the tests [source: Sala and Gobet].