There’s a few times recently where I’ve been waiting for a bus and the countdown screen has been wrong (this is in TFL land btw), in one example it turned up when according to the display it was the fourth bus due and wasn’t due for 8 minutes.
Was that near the start of the route? The system predicts services when they're on their previous trip and stood at the stand, but can't account for services leaving there early.
In another instance I was using the app on my phone, bus was showing as on time, reached the scheduled time and didn’t show up, the app changed from “due” to 29 mins, then it turned up ten minutes later whilst the app was still showing 20 mins.
Which app?