What is likely the cause of this is that the previous bus left late due to external delays you weren't aware of, and the next bus waits to leave, even if it wil

What is likely the cause of this is that the previous bus left late due to external delays you weren't aware of, and the next bus waits to leave, even if it will leave late, so that the time between the two will be roughly the same. I'm relatively confident Muni drivers are not intentionally leaving 10min early, since running an entire route 10min late makes it nearly impossible for them to use the restroom during the time between when they hit their last stop and must turn around and begin running again - a major complaint of the muni drivers' union.