Buses not in service and buses that have the engine turned off (e.g. for the driver to make a pitstop) fall off the tracker. Buses that idle at their "reset stop" show up as DLY. Sometimes when a bus turns back on, the GPS fails to connect and it falls off the tracker. Knowing the schedule is imperative and be at stops ~10min before a scheduled bus as there are some occasions when drivers run a little fast (or start late because they know they will make up time).

In short, the app is more data, but much like the schedule, it's imperfect data. I've used the bus system since the revamp a few years ago for >50% of my in-town transit to various places, and that's what I've learned.