Sometimes the bus has a mechanical fault. As it can still move
albeit at a slow speed, the driver may have been instructed to
continue.
Also it they arrive at the interchange too early, they must be
speeding. Too late, they are slack. It has to be just
right.
Bus drivers are not a special species of human beings. They are
also like you and me. They also use their own logic when
working.
You never try to be smart, at your workplace?