It's directly causal, algo's fighting for better position directly decrease the spread. Competition has the effect of removing unnecessary markup as the next gu

It's directly causal, algo's fighting for better position directly decrease the spread. Competition has the effect of removing unnecessary markup as the next guy who sells just a bit cheaper or offers just a bit more wins until the price is as close as possible to barely profitable. Humans are too slow to play such a tight game, computers aren't.