Tips days to 3 weeks later

Any clue why tips show up a few days to a few weeks later? Is it just taking Uber that long to process - Or does the PAX just get a reminder and they put it on that late? Most of the tips I see come right away but almost every day I’ll get a random one from many days ago. I could understand for business travel when someone is doing their expense report and has to go by into my trips to expense, but I know a bunch of these are not business travel.