I am familiar with the GPS tracker installed in vehicle. This is how it works.
You can set the tracker to get the GPS location at every interval (say 1 minute) and send the data to the server. The server will know the location of each vehicle at that time interval.
This is the data used to tell the commuters about the expected arrival time of the bus.
Some of the trackers in the buses are spoiled and do not send their location to the server. The server does not know that the bus exist. I describe it as a "ghost bus".
If the mobile app says that the next bus 163 will arrive in 20 mins, we may get the "ghost bus" arriving in 10 min. As the server does not know this bus, it is reporting the next bus that is arriving 10 mins later.
I have noticed the incidences of these "ghost buses" for a few years.
If the bus company or Land Transport Authority bother to look at all the buses that report their location, they will find out the buses that do not report their locations, because of a failed tracker. They can replace the tracker immediately. The tracker cost only $100.
It is a shame that the people in charge are not paying attention to this matter. It is a shameful state of affairs.