Each day, I will catch either Bus A or Bus B on my journey home from work. Both take the same route, run every ten minutes, and take the same time to reach my destination. Neither is busier, cheaper or less comfortable, and I have no preference for one over the other. 

I finish work at different times every day, somewhere between 4-6pm. I hustle to the bus stop and jump on whichever bus is there. 

And yet 90% of the time, I take Bus B. Why?