I wouldn't be surprised if it's tuned based on what people tap after a query, popularity stats generally, or other similar stats, and so they rebuild it only every few days after accumulating more stats.

Establishing popularity at a point in time requires a window of time.

If I had to guess, they pick a window of a few days because if you did it every minute or even every hour, there might be wild fluctuation in the long tail of apps, depending on whether 8 or 0 people happened to download it in that time.

You could fix this by making window size inversely proportional to short-term popularity and running every 15 min, but that's just more effort to reason about and code and debug. It seems like something they absolutely should do, but I can see why it would be pushed down the backlog.