This is something I learned today from the bus151 forum!:)
"For instance, each time you board the bus, the server recognizes that the bus has one more passenger (+1), each alighting passenger will -1. So for a typical bus capacity of 80(?), by the time say the counter for that bus reaches +60, the system will think that the bus is reaching full capacity, and will call for the next bus to depart from terminal to save the rest who are stranded. However, for each alighting passenger who doesn't tap out, the counter doesn't change (0). The system will still think the bus is packed even when physically it's not. It's further aggravated by the fact that 151 serves many schools, means there are more concession passes, who might not tap out. So adding the peak hour jam explanation in the quote above, quite easy for me to understand why 4 buses can come back to back, with the last one really empty. When the peak hour really stretches, that empty bus which is supposed to leave later would have traveled half the journey, so the terminal would be short of bus, causing a delay in the original frequency. Then causing a further delay when the next available bus comes, people tap in but not tap out, system thinking bus overloaded, system brings next bus forward, short of bus, frequency delay, ..."

Wow. Didn't know such a system exists. so cool.