Why do some journeys never seem to track on the app?

I've been trying to work out which depots runs all the misc works and scholars, and certain ones never seem to appear on the app every day, the same journeys each time.

Some of the morning 90s never appear neither do the same couple of short 94s. Emmanuel College always seems to be one bus short, always the same one. The 525 never appears. It can't be the same actual vehicles each time.