Last time I took bus at 7am to school, and I was not late. Another time, I took it at 7:15am, and I was late.
See, the difference of 15min can cause a huge different.
Then I realized that if near the start of the my journey is not smooth(traffic jam), there is a high possibility of a 'build up' effect. Which means you'll be more late.
So usually I'll take bus at 7am because at this time my avenue wasn't jam at that time, so my bus can travel a bit faster. But at 7:15am, the whole avenue is jammed, more like a avenue-lock. Speed were snail snow.