Something that the article missed is that late buses are compounded. As the delay causes more people to wait, those people getting on delay the bus even further, that's where the adage "wait ages for a bus, then two come at once" stems from. Even if the bus behind it is also delayed, it makes up time as the previous bus has scoped up all passengers. Occationally buses will divert/ terminate to rebalance the timetable.
I was recently in hospital, where the wifi wasn't great so I spent a lot of time looking out the window, and onto a busy junction. During the later part of rush hour buses often arrived together, even saw three of the same bus arrived together (only a couple vehicles separating the last bus) at one point! (like I said, bad wifi) It was staggering seeing so many ambulances arrive during the weekend, they often arrive together as the first ambulance clears a path (like in Die Hard 3).
You also see that on the tube, when people are fighting to get on a train (delaying it further) then the one following a couple of minutes behind is largely empty, it's even a little more pronounced as trains can't leapfrog each other to spread the load.